The Don Quixote that Corso buys at the beginning of the film is the very famous Joaquin Ibarra edition, published in 1780, which is generally considered as the best and most beautiful one. He also mentions the Hypnerotomachia Poliphili (Venice, 1499) printed by Francesco Colonna, dubbed "the most beautiful book ever printed". You can take a look at its amazing illustrations on the net.
The two booksellers Corso encounters in Toledo are actually the same actor, José López Rodero. Writer/director/producer Roman Polanski used a motion capture rig to use the same actor twice. The same man appears again later, playing two workmen cleaning out the bookstore. Rodero was an assistant director and production manager, not a professional actor. He was hesitant to accept these multiple parts.
All of the engravings appearing in "The Nine Gates" were commissioned for the novel (and not the movie) by author Arturo Perez Reverte himself. The only exception is the one showing the Girl riding a dragon, which was altered to make it similar to the actress' face.
Chateau Puivert (the Ninth Gate) is called The Devil's Tower. It was used by the Cathars to defend themselves during the Albigensian crusades in the 13th century. The Cathars were gnostic Christians but Pope Innocent III considered them to be devil worshippers.
The New York sequences were shot in Paris as Roman Polanski could not set foot into America because he was still wanted for his 1977 sex crime charges. The exterior locations were redressed with American-related details and the skyscraper seen in the opening and in Balkan's office were Translite material taken from Manhattan.
When Corso is first seen researching on "The Nine Gates" in the library, the cover page of the book shows the name of the author with the Latin phrase "Cum superiorum privilegio veniaque" (With Permission and License of One's Superiors) underneath it. In the novel, it is explained that this formula was used to prove the book in question was not infringing the teachings of the Church. However, the book was indeed forbidden, hinting that the author was acknowledging someone else -and not the Church- as his superiors.
Balkan explains to Corso that the Ninth Gate will conjure the Devil. What he doesn't tell him is that according to the inscription, a deal will then be struck. The Devil will agree to do or supply anything for a period of twenty years. After that, he takes possession of the person's soul. Furthermore, that person must then spend eternity in Hell cursing God (which rules out any chance of redemption).
The film has a different title from its source novel, "The Club Dumas", because the novel centers around a secret society obsessed with serialized novels, such as those written by Alexandre Dumas père.
The car that Corso and The Girl drive in France is a Dodge Viper. The model name is Viper (a venomous serpent) as Lucifer in Genesis was depicted and the car itself is painted red, a fitting color for the devil.
The trivia items below may give away important plot points.
The complete engraving titles in Torchia's Nine Gates are as follows: 1. SILENTIUM EST AUREUM - Silence is Golden. 2. CLAUSAE PATENT - They open that which is closed. 3. VERBUM DIMISSUM CUSTODIAT ARCANUM - The lost word keeps the secret. 4. FORTUNA NON OMNIBUS AEQUE - Fate is not the same for all. 5. FRUSTRA - In Vain. 6. DITESCO MORI - I am enriched by death. 7. DISCIPLUS POTIOR MAGISTRO - The disciple surpasses the master. 8. VICTA JACET VIRTUS - Virtue Lies Defeated. 9. NUNC SCIO TENEBRIS LUX - Now I know that from darkness comes light.
When Balkan has all nine woodcuts laid out, we know that one is a forgery. Of the rest as visible on screen only two are definitely the 'LCF' versions of the illustrations. The remaining six are all the 'AT' versions.