It may be a lost cause asking about a plot element in the Big Sleep (1946), one of the more convoluted and confusing movies ever made. However, my question is what is the function, if any of the ciphers. There are two scenes involving ciphers. Marlow recovers a book of Geiger's containing ciphers from rental house, and then later is seen attempting to decipher the codes in the book when Bernie walks in on him, at 2am in the morning. After that, no more mention is made of the ciphers. Is this something from the novel that was just dropped out? What is the purpose of the ciphers?
After that, no more mention is made of the ciphers.
Technically that's not quite right.
Although not specifically referenced as a cipher the whole point is that the book is a coded list of Geiger's clients (either for blackmail or the pornography).
Although Marlowe does not break the code, he has possession of the list and it's referenced in the scene where Marlowe and Joe Brody have a confrontation.
MARLOWE Joe. You got Geiger's stuff. I got his sucker list. Don't you think we ought to talk things over? Now!
Hence Marlowe can use the list (even if he cant' read it) as leverage for what he wants.