While I haven't read every book on aikido, I've covered a fair share of them that were published in English.
The only "reading" that I have done that has covered internal training to some depth, that has had exercises, hints, theory, and generally been written by someone who has a solid background ... has been here at Aikiweb.
If anyone really wants to put a price to a "book" that has some phenomenal information about aiki, internal training and skills, then one should "buy" an Aikiweb subscription. For the low price of, I think $9 or $10, you can support Aikiweb.
I don't know of any book out there that is that cheap and has that much information. No, it isn't handed to you by chapter and verse. Yes, there's a lot of posts to wade through. In the end, though, Aikiweb is the gold at the end of the rainbow.