Your lack about what gymnastics is and how complicated it is...is to blame. It all might be your true interest in incorporating gymnastics into whatever you do. It's not like many people truly care and in general they just take bits and pieces and incorporate it into what they do.
Modern day gymnastics is far more complicated than barbell lifting and olympic lifting. Look at how many events there are, how many moves there are, and for each move there are countless drills.
Hand-balancing is an entire art of it's own. Very few gymnasts can compete their handstands with an expert hand balancer.
The Navy guide to gymnastics and tumbling is invaluable. Still, it's varied as far as content usefulness when it comes to modern day gymnastics. Remember, I'm a gymnastics coach and old stuff is great ( I have an awesome near 300 page book on gymnastics from the early 70's but it doesn't cover a lot of modern stuff or it's training ) but it's not necessarily relevant nor is that book is thorough for mastery of certain elements ( I don't remember seeing it go into developing planche, lever, manna, etc )
It does not fit in the bill in developing proper basics for many of the movements and many of the CFJ articles by Roger Harrell do a better job. or the current gymnastics seminars or certs.
Of most of the current material about modern gymnastics and developing skills in it, nothing is as complete. Most of the material out there has to do with skill building and training with bits and pieces here or there as supplemental work such as training HSPU for developing proper upper body strength towards back handsprings. Some of it will state, do L-sits...voila or practice that and show it.
Many current series on gymnastics on DVD cost much more and are still cheaper than say DVD sets on martial arts ( which are sometimes a crock and just marketing ).
Seriously, it might take 300 pages to write on everything about one event in gymnastics in details ( especially when explaining the physics and biomechanics ). It will bore you to death or fry neurons if you let it. How many publishers want to print a 1000 page book on gymnastics. How much longer would it have taken to get to market. It was already much delayed and Sommer probably pissed a lot of people in how long it took to get to market from when it was originally announced.
BtGB, being the first book and the master volume is going to be bigger than the 4 book coming after. Think of BtGB as the master volume and the rest as modules. Size and price should be smaller and the next two books are going to be released simultaneously.
To end, Coach Sommer is always presenting his materials in a similar format he has seen before him by Pavel and others. Book A for BB lifts, book B for BW exercises, book C for stretching, book D for the list goes on. I'm sure Staley and others have done so as well.