"The Frogs and Toads of North America" by Lang Elliot, Carl Gerhardt, and Carlos Davidson is pretty nice book to add to your collection. It is by no means a pocket field guide that you would want to take with you on an evening frog hunt. It is too large for that. It is very good for identifying as it has large color pictures, and covers many species. There is not much info on each frog, but of course the book is not trying to be an encyclopedia of frogs. The real gem of this book is a CD of frog calls at the back. This is great for helping you learn to identify frogs by their calls. I recommend this book for the novice or younger herpers.
