Interesting thread. It's nice to see that others share a few of one's tastes. Lucy mentioned Dickens, who is a great favorite of mine; I've read all of his novels save for 'Barnaby Rudge'. Dickens, of course, was the past master of the installment method of story-telling, which many of us have put to use here.
Steinbeck, like Dickens, sent his novels out to battle for social justice. Everyone knows of the seriousness of "The Grapes of Wrath", which, in my opinion is THE Great American Novel, but Steinbeck had a wonderful comic touch, too. "Cannery Row" is probably the best example. Don't let the mediocre film of some years ago keep you from reading it. It is a true delight.
"Huckleberry Finn", as Fox wrote, is Twain's masterpiece, but I think "Roughing It" is his most humorous book.
I could write on this subject for hours, but let me just observe that I think that reading plays is very beneficial for an aspiring writer in a couple of respects.
First, plays are virtually all dialogue, which is probably the hardest part of writing, at least for me. One so wants one's characters to be distinctive and individual, not to sound 'ordinary'.
Secondly, reading plays thoughtfully (at least those without overlong stage directions, like Shaw's or Williams') forces one to do a little set design in one's mind -- which is a very useful habit to get into when it comes time to write one's own stories.
A few particular favorites: "Death of a Salesman", "Cyrano de Bergerac", "Raisin in the Sun", the Theban trilogy, and, of course, Shakespeare, who is to writing in English what the Bible is to Christianity.
Boccaccio