The great thing about "Mastering the Rubber Guard" is that covers a lot of stuff that people don't normally associate with the Rubber Guard. 2/3 of the book covers Half Guard, Spider Web and Butterfly Guard. Even if you don't have flexibility or you're uncomfortable grabbing your leg for any reason at all, there's a lot to learn from the book.
The same can be said of "Mastering the Twister"'. Since that book teaches the top game, there's no grabbing the leg at all...which is the main thing people think of when you say you practice Rubber Guard or 10th Planet JJ. Both books are good, but "Mastering the Rubber Guard" is definitely the best place to start.
And just think, if you spend the next year or 2 learning those books, you'll only have to wait another year or two after that until Eddie releases the "Advanced Rubber Guard" book