Shady Dragon Inn (B/X)-- 9
Encyclopedia Magica (2e)--22
Creative Campaigning (2e)--20
Campaign Sourcebook and Catacomb Guide (2e)--19
Complete Guide to Villains (2e)--13
Magic Item Compendium (3e)--4
Volo's Guide to Monsters (5e)--23 Personally, I suspect it's resilience is due to the fact that it happens to be an exceptionally good book.
Not to take away from the quality of Volo's, but I also wonder how much of it's popularity is because it's the most recent edition--and edition all of us in this thread are playing, which can't be said of any of the previous editions.