Should this book be part of the canon of American literature? If so, is it due to the theme, conflict, and/or American society?
I think The Maltese Falcon has canonized itself. The novel is still in print after more than eighty years have passed and many more pretentious "literary" novels as well as their authors have been forgotten. The continuing popularity of Dashiell Hammett's hardboiled novel may be partly attributable to the great film adaptation by John Huston starring Humphrey Bogart and a near-perfect supporting cast, but Huston would never have chosen the novel in the first place, and certainly wouldn't have used Hammett's dialogue verbatim, if he hadn't recognized its merit. Some of Hammett's dialogue reads like a strange new kind of American vernacular poetry, notably Sam Spade's memorable speech to Brigid O'Shaughnessy in which he says, "When a man's partner is killed he's supposed to do something about it. It doesn't matter what you thought of him. He was your partner and you're supposed to do something about it." Or: "I'm sunk if I haven't got you to hand over to the police when they come. That's the only thing that can keep me from going down with the others."
This book should indeed be part of the canon of great American works, but not for any of the reasons suggested. It should be a highly valued work of fiction because of its style, structure, form, and general aesthetic skill, including its superb characterization and its highly memorable language. Many works -- including some of little artistic merit -- can be interesting in terms of the themes, conflicts, and depictions of American society. The Maltese Falcon deserves canonization because of the artistic craft it displays at every level.