The Big Sleep is known for its convoluted plot. During filming, allegedly neither the director nor the screenwriters knew whether chauffeur Owen Taylor was murdered or had killed himself. They sent a cable to Chandler, who told a friend in a later letter: "They sent me a wire ... asking me, and dammit I didn't know either"...More at Wikipedia.
The cinematic release of The Big Sleep is regarded as more successful than the pre-release version (see below), although some complain it is confusing and difficult to follow. This may be due in part to the omission of a long conversation between Marlowe and the Los Angeles District Attorney where facts of the case, thus far, are laid out. Yet movie-star aficionados prefer it to the film noir version because they consider the Bogart-Bacall appearances more important than a well-told story...
The Big Sleep was made in the age of Hays Office censorship, and accordingly some of the more risqué elements of the plot were either presented discreetly or done away with altogether. In the novel, the books Geiger profitably rents are pornography, then illegal and associated with organized crime. The photograph of Carmen wearing a "Chinese dress" and sitting in a "Chinese chair" alludes to that.
In the film, Joe Brody is killed by Carol Lundgren who believes he killed Geiger. In the novel, Lundgren is Geiger's homosexual lover, a detail which goes unmentioned in the film.
In the novel, Marlowe finds pornographic photographs of Carmen and later finds her naked in his bed. In the film, the photographs show Carmen was at Geiger's house when he was killed (thus possibly implicating her in his murder). The novel's nude bedroom scene in Marlowe's apartment is altered in the film to a clothed Carmen awaiting him in an armchair.