Is there any book you can read over and over again without ever getting sick of it? Do you discover something new every time you read it?
There are probably a few dozen books like that. From 'Pride and Prejudice' to 'American Gods', from non-fiction and 'The World is Flat' to chick-lit and sweet as candy novels of Meg Cabot (she won my heart when Mia had a Fiesta!Giles action figure, how can you resist that?).
But the books that always surprise me with something new? Anything by Terry Pratchett. They're always new, because there's always a new level to them whenever I reread - it's because as I get older, I get more references, and I discover new things and I understand more of the books. Not all, but more.
The most startling example, probably, would be 'Wyrd Sisters'. The first times I've read it, years ago, I didn't even get the title. I've read it in Polish, sure, so the 'wyrd' concept and wordplay was lost on me, but I didn't read much into the Macbeth reference, simply because I was a kid who never read Macbeth. Easy to assume about ninety percent of the references inside went way over my head.
I reread it later, and I got things. I got the Shakespeare stuff and I got a hundred of different little things, the quotes, the three-witches thing, the playwrighting little things...
I reread it later again, after attending a Shakespeare seminar and well, if I thought I got all the jokes and references before? Heh.
And it's like this with every novel. There's no way to find all the references and in-jokes and borrowings and puns and, well, everything, but the best part about Pratchett? It doesn't matter. It doesn't matter at all, because you're having fun on the basic narrative level even without those.
(It's more fun the second time, though, and even more fun the third. When I got the Schroedinger cat joke in Lords and Ladies? Priceless moment of laugh-out-loud. Deciphering the titles once I actually read the Soul Music in English? Amazing fun. Finding the Terminator stuff in Night Watch? Woah.)
So, yeah, best books ever.