I've read many on Alex's list and think he'sprovided a lot of variety there of works that are excellent.
I'll add Truman Capote (short stories; novellas) and Carson McCullers (The Heart Is A Lonely Hunter, specifically) to Alex's list.
Also: The dictionary. It's a great read. Column at a time; page at a time; making sure to look at definitions (all of 'em) of familiar words with sometimes surprising definitions you wouldn't have expected.
It's also good to read good writing in areas outside of pure literature. I like finding books on various topics in science at the library, picking out a stack of 'em, and often finding something that is actually well-written. It's not usually inspiring for style, but the information is good to pack away to retrieve at a later date when writing.
Biographies of your favorite writers will help, too, if you can find those that have been well-written.
One area that I've been personally disappointed in is history. I have never read a history book that kept my interest because of the convergence of so many facts and dates and names and places and events and cross-references.... I'd love to find one writer of history who could keep my interest long enough to make it past the first chapter. But I know I wouldn't look at historical novels either.
