I recently read the book so I figured I would compare and contrast with this 1975 movie, which was free-to-me on Peacock.
In very broad strokes, the plot is similar: private eye Philip Marlowe gets involved with the very large Moose Malloy, who wants to find the love of his life, Velma. Who has dropped out of sight while Moose was cooling his heels in prison for a bank robbery years back. The only clue is that Velma used to work at Florian's as a songbird. When Malloy and Marlowe visit Florian's, they discover it's now owned and operated by people of color, persons of pallor are unwelcome. Malloy leaves some death and destruction, while Marlowe continues his detective work.
It's a twisted tale of depravity, corruption, infidelity, and lies. Robert Mitchum, playing Marlowe, is disgusted but diligent. Also up for a little hanky-panky with Mrs. Grayle (Charlotte Rampling), wife of a fantastically wealthy judge. Eventually, the trail leads to a gambling boat off the coast, run by mobster Laird Brunette (Anthony Zerbe).
Sylvester Stallone is in this, playing a surly and murderous thug. Moose is played by Jack O'Halloran, whose other notable role was playing General Zod's giant flunky in the Christopher Reeve Superman movies.
A number of book characters are jettisoned in this movie, notably Anne Riordan, a writer who gets tangled up in Marlowe's life. A bunch of cops are missing. The phony psychic Dr. Amthor, is replaced by an infamous madam, Frances Amthor, who's equally dangerous. For a while.
I may not have been playing the closest attention, but I'm pretty sure the movie has a number of loose ends and unexplained crimes. Ah well.