Encyclopedia Magica (2e)--18
Creative Campaigning (2e)--16
Campaign Sourcebook and Catacomb Guide (2e)--17
Volo's Guide to Monsters (5e)--11
I hadn't read Creative Campaigning before this thread, but have now and it's a great guide to help with campaign design and flavour, better than the Campaign Sourcebook and Catacomb Guide which, while useful in places, often stated the obvious and seemed to be aimed at total beginners.