Lucky, you are so spot on. The only problem, from my understanding, is quite often, once a book hits the/a bestseller list, those rights are often picked up by one of the studios.
It reminds me of a situation I was in a couple of months ago. An actor approached me, wanting to increase production value and continue putting together some fan fiction video of a piece that hadn't yet been turned into a Hollywood production, but a studio had already picked up the rights and was flailing as it had been pushed back into development at least twice. The fact that it had a fairly large fan base (7 figured I believe) and the story was interesting and the studio was in allowing fan material to be produced to help solidify their base. In the end she got cold feet after it was painfully obvious that we'd never acquire the rights to exploit the material and pulled out. She came back with another author who was more than happy to hand over his rights and have it turned into a web series/feature film/tv/anything really... It had the fan base in the hundreds (yes, 3 figures!). The material was ordinary and no fan base to boost its appeal, it was dead in the water.