It's like Wolf said, you can't get any specifics on their wild diet, everybody just mentions leaves, blossoms, fruits, etc. but, if you do a search on the type of plants and trees that are found in their natural habitat, you get a better idea (See here:
http://animaldiversity.org/accounts/Psi ... erithacus/http://www.arkive.org/african-grey-parr ... erithacus/) - not that it's going to help you much because, at least here in the States, we don't have access to any mangrove forest or savanna woodland plants!
As to medical (physiology, etc), I doubt there are any books that talk only about African grays but you can always get the avian medicine books (several of those, do a search and take your pick).