Since my brother asked.
If you've finished John Mark Reynolds' list, and plowed through Joe Carter's, you're doing well: you're classically and contemporarily wiser than most. But there are still ten books you should read to consider yourself truly educated. In no particular order:
1. The Man Who Mistook His Wife for a Hat, Dr. Oliver Sacks
Nothing is as delicate as normalcy.
2. The Death of Artemio Cruz, Carlos Fuentes
Narratively challenging, culturally penetrating, psychologically devastating.
3. Don Quixote, Miguel de Cervantes
The novel of novels. Postmodern before modern even existed.
4. Leaves of Grass, Walt Whitman
"I am large, I contain multitudes." Quintessentially American poems by the quintessential American poet.
5. 1984, George Orwell
A book that still transcends its status as a cultural cliché.
6. A Prayer for Owen Meany, John Irving
Dickensian in scope, pathos, and its sense of destiny.
7. and 8. The Sound on the Page, Ben Yagoda; On Writing Well, William Zinsser
Even better than The Elements of Style.
9. One Flew Over the Cuckoo's Nest, Ken Kesey
Even better than the movie.
10. Black Boy, Richard Wright
There is no greater American autobiography, period.
Update: You might have noticed that Christina and Josh have also posted lists.