Which is the best Guidebook ? Hard to say, because guidebooks are so diffrent...
The Berlin Zoo Guidebooks are not bad, but the new ones are always the same guide with diffrent covers. The current guidebook, published 2007, looks nice, but its very bad, becuase to many species are missing, a lot of species has two ore more pages, it contains a lot of mistakes...Its not a good guidebook. This year, the new director of the Zoo will publish a new one, I'm curious if this guidebook will be better.
The Leipzig Zoo has a very good guidebook,they publish it in english, too. And I like the Munich Guidebook. Both guidebooks are easy to handle during the visit(but which common zoo visitor is doing that ? ), the have very good picture sand maps and they are very informative.
I like the japanese guidebooks, most of them are pretty cool. I love the current guidebook of the Okinawa churaumi aquarium with the whale sharks on the cover. Its very thick and ha sbrillant pictures, unfortunately, its in japanese, but a pictures says more than thousands words...
The Guidebooks of San Diego Zoo are great, I hope, to get the 2007 edition as soon as possible...