Here's the complete list with the number of libraries that have copies shown in parentheses.
1. The Grapes of Wrath by John Steinbeck (5,327)
2. Of Mice and Men by John Steinbeck (4,899)
3. East of Eden by John Steinbeck (4,623)
4. Cannery Row by John Steinbeck (3,722)
5. Tortilla Flat by John Steinbeck (3,401)
6. The Last Tycoon by F. Scott Fitzgerald (3,293)
7. The Maltese Falcon by Dashiell Hammett (3,044)
8. The Human Comedy by William Saroyan (2,997)
9. The Wayward Bus by John Steinbeck (2,844)
10. The Loved One by Evelyn Waugh (2,641)
11. McTeague by Frank Norris (2,453)
12. Sweet Thursday by John Steinbeck (2,438)
13. Martin Eden by Jack London (2,368)
14. The Long Valley by John Steinbeck (2,356)
15. The Octopus by Frank Norris (2,349)
16. The Big Sleep by Raymond Chandler (2,285)
17. The Dharma Bums by Jack Kerouac (2,033)
18. My Name Is Aram by William Saroyan (2,021)
19. Oil! by Upton Sinclair (1,936)
20. The Postman Always Rings Twice by James M. Cain (1,903)
21. The Iron Heel by Jack London (1,760)
22. After Many a Summer Dies the Swan by Aldous Huxley (1,757)
23. What Makes Sammy Run? by Budd Schulberg (1,688)
24. Cress Delahanty by Jessamyn West (1,552)
25. The Pastures of Heaven by John Steinbeck (1,510)
26. The Lady in the Lake by Raymond Chandler (1,506)
27. The Deer Park by Norman Mailer (1,440)
28. The Pat Hobby Stories by F. Scott Fitzgerald (1,423)
29. Farewell, My Lovely by Raymond Chandler (1,389)
30. Storm by George R. Stewart (1,316)
31. The Day of the Locust by Nathanael West (1,311)
32. The Continental Op by Dashiell Hammett (1,305)
33. Mama’s Bank Account by Kathryn Forbes (1,301)
34. The Valley of the Moon by Jack London (1,290)
35. To a God Unknown by John Steinbeck (1,282)
36. Earth Abides by George R. Stewart (1,198)
37. The Long Goodbye by Raymond Chandler (1,156)
38. If He Hollers Let Him Go by Chester Himes (1,151)
39. The Subterraneans by Jack Kerouac (1,117)
40. The High Window by Raymond Chandler (1,051)
41. The Little Sister by Raymond Chandler (1,018)
42. The Daring Young Man on the Flying Trapeze by William Saroyan (1,014)
43. The Game by Jack London (991)
44. The Big Knockover by Dashiell Hammett (954)
45. The Winning of Barbara Worth by Harold Bell Wright (951)
46. Blix by Frank Norris (932)
47. Double Indemnity by James M. Cain (931)
48. Fire by George R. Stewart (874)
49. Mildred Pierce by James M. Cain (854)
50. Ask the Dust by John Fante (832)