Harry Potter and the Half-Blood Prince

I've been on a HP reread lately, and this one has been my favorite of the bunch. I read it when it first came out, overnight, in one go, and couldn't for the life of me remember anything about it. Partly that's because not much happens, but it's an extremely interesting character study of Voldemort, Dumbledore and Snape, who've otherwise been quite 2D throughout the series. There are a few plotholes around the D-man destroying a horcrux before he knew Voldemort had made them, but you know, whatever. It's a book for kids.