Gunsmithing Tips and Projects has some excellent articles by John Bivins in it on a variety of subjects and would be highly recommended. I believe it is out of print, but probably can still be found.
I like the Gunsmith of Grenville County and never have really understood why it gets such a negative view by so many readers, but it has now unfortunately become more expensive than most would pay. Recreating the American Longrifle, and Brockway's book on recreating English Shotguns are also very good books. I have Dixons book, but don't like it as much as some of the others, but it is still good to have. A very early book called the Modern Kentucky Rifle might still be available and is a good basic outline of steps and used to be quite cheap.
Though not specific to muzzleloading arms, Howe's The Modern Gunsmith, plus the books by both Baker and Dunlap are excellent resources. I find myself constantly going back to these books for information as they are incredibly complete and useful to many branches of the trade. I have many others that make for interesting reading.
