Is there actually a book that does explain the specifics of internal training? Apart from trying mushrooms...
Where explained how to achieve internal strength, how to use it, etc...
Sure, but it's like trying to read a calculus book without knowing addition and subtraction first. For example, Shioda's aikido shugyo has some interesting things to say (whilst being pretty vague at times - c'est la vie). Ditto "Chi Kung Empowerment'. There are plenty of free resources too, such as -
Frankly though...why would you bother trying to learn from a book when you opportunity to interact with people who actually have the goods? Mike Sigman and Akuzawa (amongst others) regularly visit Germany, France etc. If you're really interested, I'd suggest going to see them rather then just trying to academically understand the topic