I'm sure everyone has their own opinion about this, but for what it's worth, here's mine:
Buchele's
Recreating the American Longrifle is a classic, but it can be hard to understand at points. (Maybe there is a later edition? I've only seen the 4th ed.) It does have some good photos of longrifles, but even the photos can be confusing, since they represent several different antique styles (schools) of gunbuilding.
Peter Alexander (who along with G. Shumway helped coauthor the 4th ed. of
Recreating the American Longrifle ) later went on to write
The Gunsmith of Grenville County. I think Peter's book is easier to understand. It is based on many years of articles that he wrote, and so it has the strength of accumulated time and lots of opportunity for editing. (Alexander's focus is on brass mounted PA style rifles, but the basic techniques are the same as you will need for building a S Mountain style.)
The Art of Building the Pennsylvania Longrifle (by Dixon Muzzleloading Shop) is full of helpful sketches, but I think it is best as a companion to the others. I wouldn't try use it by itself.
I think if I was only going to buy one book, I would probably buy Alexanders
The Gunsmith of Grenville County. It covers a huge amount of detail, and it also includes some colored photos (which can be really helpful when thinking about stain).
One other thing I would definitely do, is get a copy of Hershel House's video
Building a Kentucky Rifle (by American Pioneer Video), and watch it many times (20 might be about right, including a few times in slow motion
before cutting wood. If you are working with a kit, Hershel’s video will include a lot more than you need to start out, but it will give you a fairly comprehensive view of major elements. And the fact that it is a video will help you get a 3 dimensional feel for what he is doing. Books study alone tends to contribute to a 2 dimensional view (no joke), and rifles based on book study sometimes come out a little too flat. (I’m not making a joke here. I’m serious. Squareish wrists and forearms, in particular.)
One word of warning, though. Hershel doesn’t measure very much of anything, and he is very fast. He has the experience to work that way. Don’t let that intimidate you, and don’t let it persuade you to hurry. Take your time, and measure and compare sketches frequently. Learning to sketch out what you hope to produce in metal or wood is a very helpful technique, worth the time it takes to develop.
If you buy the House video from the publisher, it also comes with a booklet. (Many other excellent videos are also available through American Pioneer Video, but I’d start with the H. House video. It’s basic and fairly comprehensive.)
Have fun!