I would start with a simple concise book with the basic layout of assembly and a good reference book on originals so you know where you are headed with your build.
Two of my personal favorites for the first "how to book" would be Chuck Dixon's "The Art of Building the Pennsylvania Longrifle, or even "Building the Kentucky Rifle" by Jim Johnston. Both are concise and have great basic layout drawings and most importantly, present the steps in a logical, clear progression. For specific things not covered in the book you can always get good advice on this board.
For the reference on originals I would get Shumway "Rifles of Colonial America" while it is currently still in print. If you have some particular styles you are interested in, Dr. Whisker's books are great as each one focuses on a specific "school" or region and allows you to get a feel for the common things, as well as the variations, within a specific style or region.
Finally - seek out mentors - find a builder near you to bounce ideas off of and critique your work, or just to help "measure twice, cut once" before tackling some of the major steps. Go to shows, talk to good builders, and handle as many originals and good contemporary pieces as you can. Books are great, but first hand study is even better.
Guy