Books about wildlife for volunteers?

I start volunteering in a couple of weeks at a local humane society. I don't really know anything about the local wildlife (birds, ducks, possums, squirrels). I don't want a book about each species, I'd much rather have a guide to wildlife describing what they eat, their natural environment, their bone structures, common injuries, etc.

Does anyone know of a basic guide to wildlife in the Midwest?

Anything especially good for volunteers (rather than a complex veterinary handbook)?