Movies that have been noted as being similar to The Ninth Gate generally fall into three categories: movies about (1) rare books and rare book dealers, (2) religious horror, and (3) secret occult societies.
FOR MOVIES ABOUT RARE BOOKS, take a look at Cast a Deadly Spell (1991), in which a detective searches for one of the rarest books of all--the Necronomicon. In The NeverEnding Story/Die unendliche Geschichte (1984), a lonely young boy finds a rare book with unusual capabilities. In Warlock (1989), a warlock from the 16th century escapes to the 20th century in search of the three parts of the Devil's Bible. For a story about a rare book dealer, there is 84 Charing Cross Road (1987), in which a used book dealer in England strikes up long-distance friendship with a customer.
MOVIES ABOUT RELIGIOUS HORROR that have been strongly recommended as similar to The Ninth Gate include The Seventh Sign (1988), in which a pregnant woman fears that her child is somehow related to signs that the Apocalypse, as outlined in the Book of Revelation, is coming to pass. Likewise, in Rosemary's Baby (1968), a woman is impregnated by Satan in reverse parody of Christ's birth. In The Wicker Man (1973), a man encounters ancient pagan practices. Of course, there is always The Exorcist (1973), if you haven't yet seen it. Other movies about religious horror that are recommended include Jacob's Ladder (1990), Angel Heart (1987), La chiesa (1989), Lost Souls (2000), The Mephisto Waltz (1971), Der Name der Rose (1986), The Prophecy (1995), The Devil's Advocate (1997), Constantine (2005) and Stigmata (1999).
Finally, movies about SECRET OCCULT SOCIETIES that have been likened to The Ninth Gate include The Irrefutable Truth About Demons (2000), in which an anthropologist researches demon cults, The Last Wave (1977), in which a lawyer investigates the hidden society of five Aboriginials he must defend in court, and Lord of Illusions (1995), in which a private investigator investigates an evil cult. More recently, movies based on books by Dan Brown, i.e., The Da Vinci Code (2006), Angels & Demons (2009), and The Lost Symbol (due for release in 2010) also center around secret societies and "gates" or steps that must be passed through in order to solve a mystery.