[dgatwood]

In the U.S., it is legally dubious thanks to the DMCA. It is legal to create it, and it is legal to possess it, but it may or may not be legal to distribute it (as I understand it, hinging in large part on whether you can convince people that such a book constitutes a "computer program", making it qualify for a fairly significant loophole).

What you could do, though, is document how the format works, but leave out the de-DRM portions, and let somebody else outside the U.S. re-reverse-engineer the remaining bits.