If you are looking for a book on muzzle loading gun building, Peter Alexander's book "The Gunsmith of Grenville County" is probably the best book out there. But at $45, it's a llittle pricey.
Recreating the American Longrifle by William Buchele, George Shumway, Peter Alexander is good too. Not as expensive as the other book, but still kinds pricey.
"Lock, Stock, & Barrel, Volume 1" by R. H. McCrory and "The Modern Kentucky Rifle", illustrated instructions, for gun builders are inexpensive books, but IMHO, not as good as those two books mentioned above.
There are other good booklets on individual aspects of gunbuilding and restoration too.
Check out the books section on Track of the Wolf's web page for all of those mentioned and more.
J.D.