I realize that some of these have already been put out here, but I'd like to reinforce their image as great books.
Ender's Game - Possibly my favorite book of all time. It has all the great elements of a sci-fi book, and has that human relationships feel to it you find in Firefly or Serenity.
Dune/God Emperor of Dune - A great series, but it's really heavy sci-fi. If you don't like sci-fi, this probably isn't the book for you. On the other hand, it's very philosophical and has a metric ton of amazing quotes.
Those are my favorites, but anything by Orson Scott Card, Sara Douglass, John Grisham or Terry Goodkind/Pratchett is amazing.