I have a book called 'An Audubon Handbook' 'Eastern Birds', that has been the best guide to ID birds. It doesn't just have ducks. The way it is layed out makes it real easy to look up birds, using pictures. I do use it for ducks, but I also use it for doves, and other things that I see in the outdoors. Great book.

Back in the day I keep a booklet called 'DUCKS AT A DISTANCE'. Good pics and small to keep in your huntin vest. I think it was produced by the USFW.
Good luck, Tom

If u have an iPhone there is an App. called iDucks.

Ducks Unlimited has an app for android with a waterfowl id section. I keep it on my phone.

I got mine from Barnes and Noble, anything recommended here should be at your local bookstore. I recommend getting one that also identifies other birds that will be around during your hunt. Geese, cranes, hawks, etc.