As the recommendations have (finally*) slowed down, I thought I'd compile the list!
I've included much of the commentary that y'all added, as well, so most of this list is comprised of YOUR WORDS. (I've used quotation marks for your bits!)
Some of the suggestions are on the younger or older end, but read-alouds are pretty flexible, so I threw them up there anyway. I didn't add most of the classics mentioned, just because the reader in question specifically tends towards a more contemporary voice, but I'm guessing that someone as into stories as she is will eventually be all over Dickens! (Who, as we know, was not afraid of the snark.)
My apologies if I missed any, and feel free to add them (or more) in the comments! A lot of the ones I haven't read are totally going on my TBR list. Because it always needs more books on it...
Thanks again for your help, and we'll totes have to do this again, eh?
Here goes (DEEP BREATH):
The Echo Falls series, by Peter Abrahams: "A very good very contemporary mystery series with ongoing location/family drama."
The Hitchhiker's Guide to the Galaxy, by Douglas Adams (x2): "That's about the time when Bill shared Hitchhiker's Guide to the Galaxy with our snarky-smart precocious daughter."
Lloyd Alexander's Vesper Holly series and Westmark, among others.
Gilda Joyce series, by Jennifer Allison (x2).
M.T. Anderson's Pals in Peril: "Will fit the bill and it is a fantastic readaloud."
The Curse Workers series, by Holly Black (x2): "Great - clever, hilarious, and very modern."
The Name of This Book is Secret, by Pseudonymous Bosch: "I actually read this one out loud myself and it was a lot of fun."
The Sweetness At The Bottom Of The Pie, by Alan Bradley (and sequels!!): "Features an 11-year-old, fiesty-as-Hades, brilliant heroine. Wonderful stories -- for adults and an intelligent kid."
Kat, Incorrigible, by Stephanie Burgis (x3): "A fun introduction to historical fiction, since it mixes in magic but stays true to period." "A younger regency protagonist with magic (and a more contemporary feel)."
Cinda Williams Chima's The Warrior Heir series.
Artemis Fowl, by Eoin Colfer (x3): "Definitely snarky!"
Gregor the Overlander series, by Suzanne Collins (x2): "Excellent pacing and character development, with a ton to talk about. And definitely has the modern-day backdrop to a fantasy/adventure." This one was recommended as a "sure thing" -- and it IS, as the reader in question has already read (and LOVED) them!
Cornelia Funke's Inkheart series. (x2)
Wise Child, by Monica Furlong: This one appears to be out of print. Sadness! Time for a re-read. "The main character is so vivid, independent, and often difficult - a realistic kid character. Also, I loved the writing, which flows as you read aloud, but is simple and crisp."
The Princess Bride, by William Goldman. (x2)
Fly by Night, by Frances Hardinge (x3): Fantasy adventure, hugely fun with the wordplay. I was so ashamed the first time this one was recommended. Because it is so very, very wonderful, and I was sad that I didn't think of it. "Add Verdigris Deep (Well Witched in the US), also by Hardinge."
The Stravaganza series, by Mary Hoffman.
Everything on a Waffle, by Polly Horvath: ACK! Polly Horvath! What was I thinking, not mentioning her? You all are the most boss blog-readers of all time.
Eva Ibbotson: I'm a big fan of Island of the Aunts. "Either her historical novels or her fantasy - the fantasy is probably aimed at a slightly younger audience but makes for great read alouds. The Secret of Platform 13 is one of my favourites." "Especially the historical fiction (such as Journey to the River Sea and The Dragonfly Pool)."
The Moomin books by Tove Jansson (x2): "I didn’t really appreciate these until I was an adult, but a more sophisticated young reader might enjoy them."
Diana Wynne Jones: In general. She's read Howl, but there are the sequels to it and so many, many more, and as any fan knows, Our Lady Diana would be hugely entertaining for the adult doing the reading as well. The Chrestomanci books were recommended by multiple commenters.
The Phantom Tollbooth, by Norton Juster (x3).
Hellspark, by Janet Kagan: "Sci fi, has one of the coolest female protagonists I've ever seen (and she's snarky, though not humorous exactly), has a mystery to be solved, and very layered and interesting anthropological points throughout (about the different species gathered together on a world and their cultural misunderstandings). I found this book fascinating as a teen and it held up beautifully on adult rereading."
Evolution of Calpurnia Tate, by Jacqueline Kelly: "It's realistic/historical. Scientifically-minded, though! Gender expectations! Research!"
Caitlin Kittredge's The Iron Thorn: "A darker steampunk/alt history adventure that has a host of creepy, smart aspects. There's a slight romance that will likely appeal to a 12 year old but it's more about action and has everything from the coolest house ever to bad bargains with fairy to a view of Boston that is all gears and asylums and well, lots of good stuff! ha! (The anti-Twilight, I think.)"
My Most Excellent Year: A Novel of Love, Mary Poppins, and Fenway Park, by Steve Kluger: "My son and I had a blast reading this. Told from three points of view, written in various styles (report to teacher, blog, emails), it's just a fun and lovely book, but with plenty in there to keep everyone interested. Baseball, musicals, movies, orphans, teens with political causes, coming-out and a very sweet boy/boy does-he-like-me romance, and plenty of funny/sarcastic stuff, including a few lines we still quote."
Skulduggery Pleasant, by Derek Landy.
Theodosia Throckmorton series, by R.L. LaFevers: "Set historically, but lovely."
A Wrinkle in Time, by Madeline L'Engle (x2).
Ella Enchanted, by Gail Carson Levine.
The Disreputable History of Frankie Landau-Banks, by E. Lockhart.
Rot & Ruin, by Jonathan Maberry: "A lot of depth and, I think, would make for a great read aloud experience."
Anne McCaffrey: I'd go with the Harper Hall books.
Robin McKinley (x4): A lot of her titles were mentioned, including The Blue Sword, The Hero and the Crown, and Beauty.
The Bloody Jack series, by L.A. Meyer (x4): "So much swashbuckling!" "Great, but the series does get a little racy as it progresses. Not anything super-questionable, but some suggestive remarks and plans for canoodling, though (as yet) it's never happened."
The Rogue Agent series, by K.E. Miller: "Good fun and would be better being read aloud."
Kiki Strike, by Kirsten Miller (x4): YES. Mysterious super-spy and delinquent Girl Scouts. What's not to love? "I imagine she might have already read it but if not will love the notion of running around in the tunnels under NYC."
L.M. Montgomery (x2): In general. The almost-12-year-old in question prefers a more contemporary flavor, but Montgomery is so surprisingly snarky and genuinely funny that I still think she'd be a good fit. (I haven't written about Anne, but I've written about Emily.)
The Forgotten Garden by Kate Morton: "A mystery that spans three generations and has strong fairy tale elements woven in."
The Dairy Queen series, by Catherine Gilbert Murdock. (x2) "That book is GREAT on audio!"
The Floating Islands, by Rachel Neumeier
The Abhorsen trilogy, by Garth Nix (x3): "Start with Sabriel, first and best in the trilogy."
Kenneth Oppel's Airborn series (x2): "So much fun."
Tamora Pierce. (x4)
Terry Pratchett's Tiffany Aching series (x4): "Rolling-on-the-floor funny, and yet so profound." "Wee Free Men is a must for reading aloud, if the reader can handle the brogue. Otherwise, get it on disc."
Going Postal, by Terry Pratchett.
The Sally Lockhart books, by Philip Pullman: Historical fiction/atmospheric mystery -- The high calibre of writing is no surprise: It's by Pullman. The His Dark Materials series was mentioned multiple times, and the requester has read them, but wants to wait a bit on them.
My Fair Godmother, by Janette Rallison.
The Larklight series by Philip Reeve: Crazypants alt-history science fiction adventure stories. "Not contemporary, but definitely snarky."
The True Meaning of Smekday by Adam Rex (x2): "Extremely snarky/silly."
The Seven Fabulous Wonders series, by Katherine Roberts.
Salman Rushdie's Haroun and the Sea of Stories: "My dad read it to me when I was around that age and I loved it! It's a book that, like Tollbooth, has so much wordplay that it really benefits from being read aloud." Seconded by me.
Septimus Heap books, by Angie Sage.
The Wednesday Wars and Okay for Now, by Gary Schmidt (x3): "It's set in 1967/8, so contemporary-ish (has a lot of what happens that year, but has a modern feel rather than a historic feel). The main character is sure his teacher hates him because he's the only one left at school Wednesday afternoons when everyone else goes to Hebrew school or Catechism, so she is torturing him with various things including Shakespeare. Definite snark throughout (though it's not comic), very interesting and layered for the parents (one of my favorite middle-grade books of all time), lots to discuss. Shakespeare, baseball, the Vietnam war, problems with other kids and family expectations, a little romance, and just terrific writing." "LOVED Okay for Now and there's a lot of snark factor there!"
Michael Scott's Secrets of the Immortal Nicholas Flamel series (x2): "Fast-paced, contemporary + fantasy/mythology elements."
A Series of Unfortunate Events, by Lemony Snicket (x2): "Take a while to get going but the series gets pretty complex in the second half, and there's a lot of literary references that a precocious/snarky kid would probably get a kick out of." "Exceptional read alouds -- funny, thrilling and quite snarky, with the most charming narrative voice. (And an excellent female protagonist.) And she will enhance her vocabulary!"
The Enola Holmes series, by Nancy Springer (x4): Sherlock Holmes' little sister! "Enola Holmes is historical fiction, but I think Enola is a very modern girl and the books have a wicked sense of humour." "I second Enola Holmes."
The Emerald Atlas, by John Stephens (x2).
Mysterious Benedict Society, by Trenton Lee Stewart.
The Bartimaeus Trilogy, by Jonathan Stroud (x2): Historical fiction/fantasy/genie! -- Fabulous use of language, hilariously original narrative voice and loads of footnotes.
The Museum of Thieves, by Lian Tanner.
Megan Whalen Turner's Queen's Thief books. (x6)
The Cronus Chronicles, by Anne Ursu
The Girl Who Circumnavigated Fairyland in a Ship of Her Own Making, by Catherynne Valente (x2): "It is absolutely FANTASTIC in so many ways and would make an AWESOME read-aloud." "It was far more fun and cheekier than I expected. And lots of snark plus a wicked spin at the end."
The Sammy Keyes mysteries, by Wendelin Van Draanan: Hard-boiled middle schooler. YES, YES, YES!! Why did I not think of these?
Scott Westerfeld's Leviathan trilogy (x3): "It's steampunk but it has a very fiesty heroine." And Uglies (x2). And Midnighters.
Mistress Masham’s Repose, by T. H. White.
Ysabeau Wilce's Flora Segunda books.
Frontier Magic trilogy by Patricia C. Wrede: I totally second this one. Alt-history/fantasy/frontier/funny/family story.
Dealing with Dragons series by Patricia Wrede (x2): This comic fantasy series riffs on traditional fairy tales, and is another one that I'm kicking myself for not recommending in the first place. I think it'd be a PERFECT fit if she hasn't already read it! "On the lighter side, but the whole series would make hilarious read-alouds."
Sorcery & Cecilia series, by Patricia Wrede and Catherine Stevermer: "Mixes regency period with magic and mystery and a good deal of humor. Very contemporary-feeling young women, considering the restraints of the period."
Lisa Yee's Millicent Min, Girl Genius; Stanford Wong Flunks Big-Time; So Totally Emily Ebers: "Funny trilogy with the Rashomon-like element that all three books take place at the same time, each from a different person's point of view."
*Holy cow, guys, you are AWESOME.