The book search engine's annual report tracks the 100 most searched-for out-of-print titles.
Promise Me Tomorrow, by US romance author Nora Roberts, came second.
BookFinder.com said the prolific author "actively persuaded her loyal fan base to avoid this early novel for years" and hoped the 1984 work would never be republished.
"We suspect a used bookshop is the only place you will find this title," it added.
BOOKFINDER'S TOP 10 MOST WANTED BOOKS1. Sex - Madonna
2. Promise Me Tomorrow - Nora Roberts
3. Rage - Stephen King (as Richard Bachman)
4. My Pretty Pony - Stephen King
5. In a Dark Place: The Story of a True Haunting - Ray Garton
6. Codex Seraphinianus - Luigi Serafini
7. Man in Black - Johnny Cash
8. Marilyn: A Biography - Norman Mailer
9. Arithmetic Progress Papers - H Henry Thomas
10. Mandingo - Kyle Onstott
The Top 10 also features Johnny Cash's memoir Man In Black and a Marilyn Monroe biography by Norman Mailer.
Other notable books on the list include Fly Fishing: Memories of Angling Days by JR Hartley - a title inspired by a famous 1980s Yellow Pages advert.
Among the new entries tin this year's Top 100 was knitting expert Alice Starmore's Tudor Roses, a book featuring "sweater designs inspired by the Tudor royals".
According to BookFinder.com, Sarah Bradford's 1989 biography of George VI, The Reluctant King, had been in "steady demand" following the release of Colin Firth film The King's Speech.