I have the Audubon guide to reptiles and I REALLY do not like it. Not complete enough.
I have iBird Pro and Peterson Birds of North America. I like both of those. I think iBird Pro is more intuitive. Peterson is, however, like the book so you can view birds according to type eg. Waders, Woodpeckers, Chickenlike Birds, etc.
My friend the ornithologist swears by Sibley, the book, and if I had a spare $30 I'd probably get that one as well.
I'm sure iBird has a free version you can evaluate, and Peterson Feeder Birds is either free or cheap. I got that first before buying the complete guide and liked it well enough to buy the big one.