Faking
The most recent editorial on the current spate of fake memoirs put me in mind of John Henry Newman, who had this to say about Maria Monk:
...A writer of name, of character, of honour, of gentleman-like feeling, who has the entrée of the first and most intellectual circles of the metropolis, and is the friend of the first Protestant ecclesiastics of his day, records his testimony against Catholicism; it is in the main true, and it fails:—a worthless stroller gets her own testimony put into writing; it is a heap of fables, and it triumphantly succeeds. Let, then, the Protestant public be itself the judge:—its preference of Maria Monk to Blanco White reveals a great fact;—truth is not equal to the exigencies of the Protestant cause; falsehood is its best friend.
Newman hits on the market question: fake "autobiographies" can serve any number of purposes (including just plain faking), but a smash hit like Maria Monk fills--or perhaps produces--a definite niche. Moreover, depending on the niche, such fakes may quickly replicate themselves beyond the control of any fact-checking debunker; Maria Monk, for example, had already been exposed in Newman's time, and yet the book still has some purchase in extreme Protestant circles.
I'm permanently skeptical about anything advertised as an "ex- " memoir, no matter what the author is supposed to be "ex- " from.
(Joseph Blanco White [a.k.a. José María Blanco y Crespo] had been a friend of Newman's at Oxford. The book Newman discusses here is The Poor Man's Preservative Against Popery.)