I've pulled up a number of free titles on my Nook including those of Ogden, La Branche, Clark, Mary Orvis, H.G. McClelland and George Parker Holden.

McClelland's How to Tie Flies for Trout and Holden's Stream Craft were both important in the development of the Catskill Dry.

For a couple of bucks, I got Eric Leiser's Complete Book of Fly Tying.