Can anyone recommend some "heavy" books?

I looove to read - it's as natural and daily as eating. But lately I felt like I've been reading too much fluff; I often pick up books just for entertainment, nothing particularly weighty or meaty, nothing that really makes you think. Could anyone recommend some books that deal with some deep issues, maybe ones that aren't very well-known? I've read some of the classics (To Kill a Mockingbird, Anne of Green Gables, Call of the Wild, Huckleberry Finn, Little Women) and enjoyed them greatly. But beyond Jane Austen and John Steinbeck's books, which I haven't gotten to yet, I can't really think of any other books to check out that aren't just cotton candy for the brain.