Your ISP’s knowledge base will have factors behind it. Do you use their provided router? If so, they can easily have more data about your internet usage than if you’d use your own. As far as modem’s go, I don’t think it can provide as much data compared to a router, but it’s still worth the cost to obtain more control of your information.
Overdrive seems like data middle man to me, give LibriVox and/or The-Eye a try instead. Hopefully one or both have all the books without the privacy worry.