I wouldn't call this isn't an exploit. This video is just going off to play which winds up really doing nothing but reloading a merchant's inventory.
An exploit would be along the lines of buying and using the book. Saving then Exiting and reloading the game and buying the book again, then repeating that over and over.
Having to level in between each use isn't an exploit.
You have a character modifying book that is not supposed to respawn. We know this because the developers have stated it. However, it is respawning.
That is a bug.
Knowing it is a bug and purchasing additional copies of the book that you know shouldn't be there is exploiting that bug.
How much that really matters in a single player game that can be modded is debatable, but it still is what it is. You can call it abusing a known bug, exploiting a known bug, taking advantage of a known bug, intentionally reaping the benefits of a known bug, etc.. it's all the same. Nobody can claim they're just "playing the game as it was intended" once they know the bugged behavior is not actually intended.
