Probably the best of the bunch and a good primer on the topic is "The 'Black Mask' School" by Edgar-winning UCLA Professor Philip Durham, focusing on the origins and contents of the eponymous pulp magazine.
Some essays are insightful, others veer off into academese, others display a surprising distaste for the whole topic:
"Although the novel is atrociously written, with ... a tone I can describe only as illiterate archness, it does contain some of the important elements of gangster fiction: an Italian hero, an unbelievable amount of brutality ..., quite a bit of very rapid and decidedly unexciting sex, a Robin Hood sort of romanticism, and some fairly knowledgeable accounts of the methods of criminals."
Of a different novel: "Chase apparently took all the elements he found striking in gangster fiction and magnified them as far as his imagination and the censors would allow; the result is one of the rarest of rare birds, a truly horrible book."
One interesting insight that caught me eye is the epigraph to an essay on Hammett, quoted from Angus Fletcher's Allegory: "[The 'daemonic agent'] will act as if possessed ... He will act part way between the human and divine spheres, touching on both, which suggests that he can be used for the model romantic hero, since romance allows its heroes both human interest and divine power. His essentially energic character will delight the reader with an appearance of unadulterated power. Like a Machiavellian prince, the allegorical hero can act free of the usual moral restraints, even when he is acting morally, since he is moral only in the interests of his power over other men. This sort of action has a crude fascination for us all; it impels us to read the detective story, the western, the saga of space exploration and interplanetary travel."