Every now and then, a single volume combines two or more of Hammett's novels under one cover.  Here are but some of them:

Dashiell Hammett Omnibus, Knopf, 1935 (includes Red Harvest, The Dain Curse, The Maltese Falcon )

The Complete Dashiell Hammett, Knopf, 1942 (all five novels, in reverse order of publication)

Dashiell Hammett Mystery Omnibus, World, 1944 (The Maltese Falcon, The Glass Key )


Dashiell Hammett Omnibus, Cassell, 1950 (all five novels, plus several short stories)

The Novels of Dashiell Hammett, Knopf, 1965: (all five novels)

Five Complete Novels, Avenell, 1980 (all five novels)

Five Complete Novels, Wings, 1980 (all five novels)

The Four Great Novels, Picador, 1984 (Red Harvest, The Dain Curse, The Maltese Falcon, The Glass Key)

Complete Novels, Library of America, 1999 (all five novels)

Dashiell Hammett, Everyman's Library, 2000 (The Maltese Falcon, The Thin Man, The Glass Key)

Here's a rare one: Dashiell Hammett: Selected Detective Prose, published in Russia in 1985. It includes (in English) The Glass Key, The Thin Man, and two short stories, "Fly Paper," and "The Gatewood Caper."  There's also a critical essay in Russian, and an English-Russian slang glossary.