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Rob D.This doesn't come close to qualifying as the best baseball fiction book, but The Universal Baseball Association, Inc., J. Henry Waugh, Prop. by Robert Coover (1968) is a wonderful read, even more so if you've ever been addicted to playing Strat-o-Matic or APBA. You can knock it out in 2-3 days without really trying.
For a synopsis, here's Wikipedia's entry on the book.