re: What are your top 5 western books?

The Virginian
Last of the Plainsmen (better Zane Grey book than ROTPS IYAM)
NCFOM
Blood Meridian

There's a nonfiction book I'd put above all of them: We Pointed Them North. In the 1930's, historian Helena Huntington Smith sat down with E.C. "Teddy Blue" Abbott, a Montana cowboy then in his seventies. Larry McMurtry used it as source material for Lonesome Dove. If you're interested in Western history, it's a must read.

ETA: in fact, there's so much great western nonfiction out there, that I usually prefer it to fiction.