Roots by Alex Haley.
Alex Haley doesn't know what his "roots" are, so he had to make it all up from some oral family traditions.
And a long-suppressed tape of the famous session in which Haley " found" Kunta Kinte through the recitation of an African "griot" proves that, as BBC producer James Kent noted, "the villagers [were] threatened by members of Haley's party.~
These turn out to be senior government officials desperate to ensure that things go smoothly."
Go Ask Alice - A Real Diary by "Anonymous" (i.e., Beatrice Sparks).
After the success of Go Ask Alice, Beatrice Sparks just "happened" to come across six other 'diaries' that she was able to mass-produce in book form. Interesting...
Everything written by Carlos Castaneda.
Got any more that I'm not aware of? Researching this on Google doesn't count! I want to know about other fake non-fiction novels (etc) that were exposed as fraudulent after publication.