CoreyJoe is right on. I've found the best two books are the Buckskinning volumes II and VI. Not only good info on building a very sound powderhorn, but some info on the different 'schools' of styles (e.g., screwtips) and basic history is invaluable. From a perspective on how to make your powder horn 'look good', get Sibleys book too. They also have information that's valuable in building your horn too, especially if you want to engrave the horn with great pictures, in color.
If you choose to advance yourself and for studying older horns, 3 books standout because of their history and outstanding color pictures that really shine on the details. You may want to get Grinsdale's recent book, William Guthman's book, 'Drums A Beating, Sounding the Trumpet', and Dresslar's book, 'The Engraved Powder Horn'. All three I constantly refer to.
IMHO.
Gary