Included, not sold separately here
The device guide for your tablet comes with it.
A book on the device itself rather than on the files you put on it: the settings that are hard to undo, every route for getting files onto it and the ceilings they hit, what a subscription really buys, and what to try first when something has gone wrong. Chapters: 17 on the reMarkable, 15 on the Kindle Scribe, 14 on the BOOX.
One edition per device, and the one you get is the one for the tablet you picked above, a $19 book on its own. Nothing to choose at checkout and nothing to write in for: the picker in the hero already said which tablet you are on.