Everybody has their own preferences, but I would get the most straightforward instructional book on basic assembly and building and spend the other money on a book or two with good photos of originals in the style you are interested in.
For basic assembly, to me, Chuck Dixon's "Art of Recreating the Pennsylvania Longrifle" and Jim Johnston's "Buidling the Kentucky Rifle" are a lot of bang for the buck and cover the basic layout, and have clear, concise drawings. The Alexander book has a lot of good information, but to me some of the good basic stuff you really need as a first time builder gets lost in the weeds along the way, especially if you are starting with a precarved stock. The Hershel House video and companion book are good - I would say they would compliment one of the more basic books nicely.
For originals, Jerry Noble's books are about the only resource out there dedicated to southern Appalachian mountain rifles, if that is what you are wanting to build. But even if you don't want to build an early rifle, the Shumway "Rifles of Colonial America" vol. 1 and 2 are essential to any longrifle builder, I think, since they have dimensions and can teach you a lot about proportion and layout and if you get them now you will thank yourself someday when they are out of print again!
The reference books are lacking a little with tips on finishes etc. A lot of good info on this sort of stuff can be found in the archives on this website.
Finally - there are great shows that you can attend and there are many collectors who are generous enough to bring their originals and display them, and will let you study these pieces closely. Always ask of course, but you will find that if you go to a few shows you will find yourself face to face with that original you've seen in the books and want to copy. Nothing beats being able to handle one and see all angles, the balance, etc. They always look different in person than on a book's pages!
Good luck
Guy