A review by sherwoodreads
Memoirs of a Woman of Pleasure by John Cleland

This is a peculiar book, more interesting for its ideas about relations between genders, especially in houses where sex was for pay, and for its ideas about the psychology of sex, than for its eroticism. (Or its ideas about how only a male can give a woman true pleasure.)

The fact that it stayed in print for longer than most other books indicate not just the perennial popularity of porn (which actually comes and goes) but suggests to me that some used it as a sex manual.