The BookSeekers is geared specifically to finding children's books, specifically, in their words, "a discovery website for kids’ books which seeks to help you to navigate through the huge choice of books for kids – from toddlers to teens - to find the next great book to read".
As I've done in the past, I ran Howl's Moving Castle and The Book Thief through the engine.
For fans of Howl, BookSeekers recommends: Charmed Life. Which I feel is weaksauce, because A) only one title? and B) that one title is ALSO by DWJ?
For fans of The Book Thief, BookSeekers recommends: I Am the Messenger and Boy in the Striped Pajamas. Weak (same author) and barf (I understand why it came up, but man, I do dislike that book).
Anyway, I'll continue playing with it—it looks like there are other ways of using it and there are lots of booklists to comb through—and it's a pretty new site, so they'll probably continue to add to the database and tweak it and whatnot.