Have you considered either a) if they allow a binder then photocopying the book into it or b) taking the second book if the first binders are not allowed? For fossils, my school would just find every single good fossil book and photocopy all relevant pages into the binder, and put in a lot of internet info. It worked extremely well, even though the binder was extremely heavy.Thanks for the advice. In that case, I'm probably going to get Sibley. I think Audobon is the only guide that has all the information however it is bad for competition because of its format and it is split into two books. Sibley has nearly as much information as Audobon and it is all in one book. National Geographic looks good too with information however it doesn't have range maps which is a big disadvantage. National Wildlife Federation doesn't much information to begin with.
If they don't allow a binder, and instead do a second book like for ornithology, I'm going to use Sibley, and for the second book either Audubon or a dendrology textbook.