All of the engravings appearing in "The Nine Gates" were commissioned for the novel (and not the movie) by author Arturo Pérez-Reverte himself. The only exception is the one showing the woman riding a dragon, which was altered to make her face similar to the The Girl and the burning castle to match the location used.
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". These amazing illustrations can be viewed on the internet.
There is a whole subplot in the novel regarding the French hotel's receptionist and his past interactions with Corso. In the film, the hotel manager is named Grübber and has a half inch middle mustache like Adolf Hitler's. This is a reference to the book, where he is described as an extremely punctilious, detailed man who wears his uniform with military discipline, and it is heavily implied that he is a former Nazi and possibly a war criminal who changed his identity, and also owes Corso some sort of favor. This explains his attitude towards him in the film, and why he does not question his extremely suspicious requests.
The bag that Dean Corso carries throughout the film is a small bag carried by French soldiers around 1935. It was used to carry ammunition and other small items. It is called a Musette MLE 35.