Sounds like a great opportunity for content marketing!
Medium is already home to tons of book reviews, and this new acquisition will make it easier for readers to buy books after reading about them here. (I’m guessing this is what Ev Williams has in mind based on his initial communication).
I recommend that authors ask our community for to publish both in-depth and short form reviews here on Medium. But first, authors should compose a brief introduction to the published books themselves. Include a call-to-action at the end of your story to let readers know where they can find your book(s) currently, and ask them nicely to share their review or testimonial here on Medium.
We won’t know exactly how this new project will unfold (Medium doesn’t know yet either), but reviews and testimonials are low-hanging fruit during this early development stage.
This is our chance to breath new life into all those wonderful ebooks collecting digital dust online!