This one is going to be a very subjective list. My list would include books by Dashiel Hammett, Raymond Chandler, Ross Macdonald, James Crumley, and I am afraid these writers would alone account for a bulk of items in any good private detective fiction list.  Dick Lochte, novelist, short story writer and the current President of The Private Eye Writers of America. picks out his list of top 20 Private Eye novels and he too says that books by Hammett et all would complete his list. So he includes just one book from each author instead:

His list: [The highlighted authors are my favorite]

1. Charles E. Alverson - Goodey’s Last Stand
2. Lawrence Block – Eight Million Ways to Die
3. Howard Browne – The Taste of Ashes
4. Raymond Chandler – The Long Goodbye
5. Robert Crais – L.A. Requiem
6. James Crumley – The Last Good Kiss
7. Stanley Ellin – The Eighth Circle
8. Earl W. Emerson – The Rainy City
9. Loren D. Estleman - Every Brilliant Eye
10. Joe Gores – Dead Skip
11. Sue Grafton – ‘K’ Is For Killer
12. Dashiell Hammett – The Maltese Falcon
13. Arthur Lyons – Hard Trade
14. Ross Macdonald – The Way Some People Die
15. Walter Mosley – Devil in a Blue Dress
16. Warren Murphy – Trace # 1
17. Robert B. Parker – The Judas Goat
18. T. Jefferson Parker – Silent Joe
19. Brad Solomon – The Open Shadow
20. Jonathan Valin – Day of Wrath

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