One of the books about hardboiled fiction that rarely gets mentioned is Hardboiled America by Geoffrey O'Brien. He's a literary writer of real distinction (as well as the editor of The Library of America) but he's not slumming. He loves and understands the material. And he writes with real elegance.
His assessment of such major writers as Hammett, Chandler, Woolrich, Gardner are very much his own, and all the more fascinating because of it. He also takes the time to illustrate how literary fiction influenced hardboiled and how hardboiled influenced literary and mainstream.
For me he's at his best with the second generation of hardboiled writers, namely the Gold Medal girls and boys and how they spun off into Lion, Graphic, Ace, etc. I wish he wasn't so dismissive of John D. MacDonald. Here he takes the familiar path of the neo-noir critics who complain that JDM wasn't tough enough in his viewpoint. Most of his books concern middle class or working class men and women confronting crime. They're not gumshoes, they're not criminals. They bring their manners and mores with them when they try to extricate themselves from their problems. It's not that he isn't hardboiled; it's that he doesn't use all the cliches of hardboiled.
O'Brien shines when discussing Day Keene, Harry Whittington and, especially, Charles Williams. In fact I think his piece on Williams is definitve. Hard to imagine anybody handling Williams' career any more shrewedly.
The Hardboiled Checklist at the back of the book (1929-1960) is the most intelligent, exhaustive such list I've ever seen. Makes you wish you had three lifetimes just to read every book he takes note of.
This belongs on the shelf of every hardboiled reader and writer. It doesn't get any better than this.