My all time favourite book is Speak.
I like the Divergent series, I still think it has one of the worst endings ever though.
The Stephanie Plum books are fantastic, definitely 18+
I have a "booket list" (a bucket list of books) and on it I have all of the old classics, Pride and Prejudice, A Tale of Two Cities, etc. and the Harry Potter books.
@Eyes On The Prize I love going to thrift stores for books too! They're all $.50 - $2.50 it's great! And you can get really cool special editions that aren't in regular stores.