This thread loses 80 billion points for not having a single mention of the now inaccurately named Foundation Trilogy, which has expanded to 5 books, and even more if you count the sidestories. Foundation and Empire (the second book in the series) is the best thing I've ever read.
Also, while they won't expand your vocabulary, I highly recommend the Hitchhikers' Guide to the Galaxy series by Douglas Adams and How I Conquered Your Planet and The Exploding Detective by John Swartzwelder, all of which are some of the funniest things ever.