I would imagine that getting a book published (especially if you are not already published) is quite difficult. I would try any publisher whose name you can get hold of just to increase your chances.
The other option is to self-publish, which in the age of the internet and Amazon is much easier for independent persons to do. That is what I will likely do on my Zoos Of The Southwest project. The down side, of course, is you have to pay all the printing costs up front yourself. If the book is strictly text or strictly black and white, the costs are reasonable. If you include color photos, the price jumps up.
Also, if you do include photos, you may want to get a preliminary ok from the zoo, as many zoos have a blanket statement that "commercial photography is prohibited." In the photo industry (in which I work), technically your use is editorial photography and not commercial photography so it should be exempt. But best just to get a preliminary ok and avoid any potential issues.