I look at it this way. Suppose I wrote a novel, and someone else out there typed up a set of notes telling readers which pages they should skip. Then suppose they sold that set of pages to anyone who wanted, as long as they had proof that the buyer had already bought a copy of my novel. Now, on the one hand, they're clearly not cutting into whatever amount of money I'm making from my own book. But on the other hand, they're making money off of the fact that people have bought my book. It could be argued that their set of notes is equivalent to a derivative work, such as fanfic or a role-playing game based on my novel. And I would be very upset if someone made money off of either of those things without a proper license.

I just wanted to note that the very first thing my brain flashed to here was "Cliff Notes". I am curious as to your opinion of those.