The plot keeps you wondering from one page to the next, but in the end it's hard to figure out what happened. The prose is competent but choppy and occasionally includes bright figures of speech. The characterization is based on vivid physical images, but the characters are not fully human. The dialogue tends to be more informative than interesting, but is carefully adapted fit each each character.
The setting is San Francisco of 90 years ago, which is vividly described in physical detail. You learn a lot about streets, restaurants, and the inside of certain hotels. (One of the restaurants, John's Grill, is still operating, based largely on its appearance in the novel, and mostly for tourists. It's pretty good.) If you don't know, the McGuffin is that an extremely valuable gilded and bejeweled bird was created in the Middle Ages. It has disappeared in the random passage of history, and a group of colorful characters now pursue it with greedy obsession and without conscience.
The main interest of the book lies in a dance of deception between two sociopaths. The heroine/villain, Bridgid O’Shaughnessy, never speaks a true word, and the detective, Sam Spade, is only occasionally honest, but talks less and often practices obfuscation merely by manipulating his facial expression.
Characterization is a sort of Reductio ad absurdum of the doctrine that became popular decades later in many creative writing classes, "show, don't tell.” There are only occasional brief references to mental processes, mainly of the obese brid-seaker, Mr. Guttmann. Guttmann’s fat face is several times described in grotesque, Dickensian detail. It is not clear if these descriptions are intended to be comic or not. The tightly hobbled expression of inner life forces Hammett to provide lengthy self-descriptions by Bridget O'Shaughnessy, which amount a series of mutually contradictory soliloquies, and to obsessive description of facial expressions, gestures, and body language. Sometimes these prose mimes are asked to carry more meaning than they can credibly bear, particularly eyes:
"He stood beside the fireplace and looked at her with eyes that studied, weighed, judged, her without pretense that they were not studying, weighing, judging her. She flushed slightly under the frankness of his scrutiny, but she seemed more sure of herself than before, though a becoming shyness had not left her eyes."
Someone's eyes are described on almost every page of the book.
The detective is described many times as having yellow eyes and a V-shaped mouth. I've never seen a human being with such features, which suggests he may be a space alien, but I doubt that's what Hammett intended.
Considering that, is as far as we know, the heroine is a total self-fabrication, can it be a coincidence that Hammett's main squeeze was Lillian Hellman, of whom Mary McCarthy famously quipped, "Every word she writes is a lie, including 'and' and 'the'".
Many people's memories confound the book with the 1941 movie. In plot, the movie follows the book pretty closely, with a few cuts and simplifications, but in subtle but important ways it is quite different. In particular, Humphrey Bogart plays the detective. The detective of the book is an unusually large and physically intimidating man. Bogart was five foot nine and couldn’t play it that way. Moreover, Bogart is engaging and likable, whereas the detective of the book is harsh and cold. More generally, a movie loaded with good actors has the great advantage that actors can portray facial expression and body language much more richly and subtly than the printed page, so that they make up for the handicap of “show, don't tell.” The movie also softens the mindless viciousness of the characters in some secondary ways.
I give away nothing that you don't learn in the first few pages in reporting that the detective's partner is soon bumped off and that the detective has been sleeping with his partner's wife. I wistfully imagine a different novel that begins with the detective's thoughts about his relationship with the wife, whatever it may be, and hence how he feels about his partner, and hence what caring about people in general means to him. Then, when we learn the partner has been murdered, it would mean something to us.