Some of our best birding books are now available in digital format from iTunes. Princeton University Press has published six of its recent books in ebook form. They are as good-looking in digital form as they are on paper. Katrina Van Grous’s “The Unfeathered Bird” is one of the books. Her detailed and incredible drawings of avian skeletons lose nothing in the translation. “The Crossley ID Guide” also is available, with plates shown in greater detail. Ditto “The World’s Rarest Birds.” Take a look. Princeton will be adding titles regularly to its new digital book shop.