This is the last novel Raymond Chandler wrote, or perhaps just the last Philip Marlowe novel. Likely both given that Chandler died a year after this novel's publication. I don't think it was quite so good as the four or five other Chandler novels I've read, but it is still good (orders of magnitude better than anything Dashiell Hammett ever wrote).
In this book, Marlowe is hired to track a young woman. He has been told to pick up her trail at the train station when the "Super Chief" comes in. He has no idea why. When the young woman arrives, he sees her in an argument with a young man, who appears to be blackmailing her. Marlowe has no idea why. So, they all wander down near San Diego. Marlowe discovers a couple more people are trying to keep tabs on the young woman. And so on. The driver of the story is that Marlowe can't just go off without eventually coming to an understanding as to what it's all about. People are offering him too much money for such a seemingly simple task. So, he smells a rat. As a result, he keeps coming back to the young woman, the other people following her, a couple of bodies that show up along the way, and so on. Good classic hard-boiled, noire detective fiction.