See this post for updated info. Thanks to tubemonkey 
https://www.mobileread.com/forums/sho...&postcount=215
This is my unofficial and unscientific ranking of libraries with the most unique titles. I updated the numbers from those previously posted. Some libraries added titles since the last thread. I also adjusted (hopefully) for titles with multiple formats by adding the fiction and nonfiction titles separately to get the total number. This is by no means perfect and all errors are my own. Post any errors or libraries that should be added and I'll update the list.
[Updated June 21st 2012 - previous update Jan 7th]
1. Indianapolis 40947 .........................[ +11531 titles]
2. Seattle 40589 ................................[ +7510 titles]
3. Cleveland 38493 .............................[+6273 titles]
4. Singapore 34408 .............................[ +2985 titles]
5. Hennepin County (MN) 33824 ...........[ +9815 titles]
6. Columbus (OH) 32521 .....................[ +17387 titles] *
7. Phoenix 28773 ...............................[ +4021 titles] *
8. Orange County (FL) 28400 ...............[ +3065 titles]
9. Brooklyn Public 28114 .....................[ +6502 titles]
10. New York Public 26519 ....................[ +2758 titles]
11. King County (WA) 21532 ................ [ +5827 titles]
12. Calgary 20537 .................................[ +4565 titles]
13. Lee County (FL) 19890 .....................[ +2925 titles]
14. Salt Lake County 18396 ..................[ +2551 titles]
15. Pittsburgh 17233 ...........................[ new entry ]
16. Hamilton (ON) 17091 .......................[ +2709 titles]
17. O.D.L.C. (OR) 16061 .................... ...[ +3115 titles]
18. Monmouth County (NJ) 15948 ..........[ +1484 titles]
19. Ontario 15505 ................................[ +3018 titles]
20. British Columbia 15381 ...................[ +2111 titles]
21. R.E.A.D.S. (TN) 15130 ....................[ +1360 titles]
* best approximation