How does this novel relate to the concept of Horace’s dual function of art?

Horace said art should both delight and inform. The Maltese Falcon delights by offering a suspenseful story written in a spare style with many tricks, turns, and surprises as the plot unfolds. The seamy, corrupt California setting makes a compelling backdrop for this hard-boiled crime story in which almost everyone is shown to have dirty hands. The mysterious, highly valuable falcon from the "Orient" also adds to the book's flavor. Finally, characters such as the good-looking but tainted Sam Spade and Brigid capture our imaginations.
Unlike a cozy mystery, for example, in which truth prevails, the real murderer is unveiled, and a just universe is affirmed, The Maltese Falcon informs us of the reality that justice is not always served. Spade is quite ready to throw an innocent fall guy under the bus as a murderer. This story, in which basically everyone is willing to betray everyone else, serves as a cautionary tale to the reader to be careful and to recognize that evil exists in the world.