Nick Loper titled his webinar "Side Hustling with E-books (the marketing lessons I learned from launching 6 e-books)".
When he started with just having a blog on outsourcing, he wanted to build some authority, so he wrote a book on virtual assistants. Very few sales through his site [my problem too, at this point] and low revenue from Amazon [I haven't published on Amazon yet]. Part of the impact and boost in authority came from the content and being referenced and quoted in other sites.
He wrote a second book on treadmill desks, probably a first on this topic. It was a short book, but he included some case studies from other offices who had treadmill desks. The mentions in the book gave it a bit of a built-in virality that helped at the launch.