A great book on Capote, that covers that rivalry, (between Capote and Harper Lee) - is Capote, by Gerald Clarke. Unusually fine biography. I don't believe it rumor by the way. But Capote was very annoyed that his life-long friend made such a splash with Mockingbird, which became an instant American classic.
Thanks for the info. I'll have to check out that book -- I really enjoyed Clarke's book on Judy Garland.