The very best must have book for tuning theory I've seen is this book written by Greg Banish
https://www.amazon.com/Engine-Management-Advanced-Greg-Banish/dp/1932494421
Must have for anyone wanting to learn the proper way to tune.
His other book is also worth having:
https://www.amazon.com/Designing-Tuning-High-Performance-Injection-Systems/dp/1932494901
Greg was my mentor when I designed and tuned my MAF system for the 84-89 3.2L. Greg is one of the most talented free-lance tuners in the world, often hired by FORD and GM labs for his services. He was instrumental in helping FORD with the eco-boost engine design.