This article is from the Sex Movies FAQ, by director@gti.net (The Director) with numerous contributions by others.
As far as I know, only one: 1975's Naked Came the
Stranger.
William George Ferguson says:
The (X-rated) movie was the rights-purchased, official
movie version of the book. The book had made the New
York Times bestseller list before it was revealed that
Penelope Ash was actually a group of Newsday writers
doing a parody (the one female member of the group posed
for the back cover author's photo).
After the revelation, the book reached number 1 on the
NYT bestseller list. This is, as far as I know, the only
example of a true best selling book having a hard-core
porn movie as its official movie version.
 
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