Really? I'm ashamed to say I'd never even heard of him until I read your post, Harms.
I'm not surprised. Despite receiving a fairly consistent level of acclaim throughout his career, and holding some weight afterwards (his novel THE MOVIEGOER made it onto the TIME's "greatest novels" list, the same one that features WATCHMEN), has never really had the widespread notoriety and acclaim of, say, Toni Morrison or the like.
So what exactly is so great about this guy?
Well, his beliefs about the dislocated self in the twentieth century are quite thought-provoking. All of his major works, in one way or another, come back to the state of the self and the decay of Western civilization. I think Percy's insights on this topic are not only interesting, but well-articulated, often drawing from his background in scientific study (he was a doctor before he became a writer) and his Roman Catholic background (though he's unlike any other Christian figure I'm acquainted with, and would likely enrage both conservatives and liberals with his commentary).
But all this stuff could easily be accomplished in nonfiction rather than fiction (and make no mistake, Percy's nonfiction material is quite good). But what makes him a great fiction writer? Well, not only are his novels are thinking-person's literature with a great deal of thought informing them, but they tell unusual and engaging stories ranging across a rather wide spectrum, ranging from lighter romances (THE SECOND COMING) to suspenseful thrillers (THE THANATOS SYNDROME), with elegant, witty prose, and a sea of bizarre but endearing (or at least interesting) characters. Weight
and entertainment... what more could you ask for?
Which book(s), in particular, would you recommend?
Well, you can always read THE MOVIEGOER, which is his most widely-read and acclaimed novel. But, while very good, it's not my favorite. I'm partial to LANCELOT, a darker story about a lawyer in an insane asylum. And, if it keeps up the way it's been going, LOVE IN THE RUINS might be right up there, too. But I haven't read a piece by him that hasn't been worthwhile, including his nonfiction.