In short, we work with domain/subject-matter experts to publish high-quality eBooks. Some we create ourselves, some we work with publishers who already have great content (eg, Mixergy).
Since we're a startup there's still a lot that we need to improve, but one thing we're doing is making the books more interactive, and for PG's book we'll be adding video clips from the interview. Unfortunately Kindle doesn't have an easy way to provide existing customers an update, but if you did buy the book, please email me (kevin@hyperinkpress.com) and I'll share it with you.
Also, if you have knowledge on any topic that people would pay to learn more about, and want to create a beautiful, high-quality eBook to share that knowledge, please reach out to me - we're actively recruiting authors. A big thing of ours is sharing a lot of the content for free, it's not purely altruistic since that's great for building loyalty and traffic too. Thanks!
Kevin, Founder of Hyperink
PS Right now hyperink.com has almost nothing on it, we'll be launching a marketplace for our eBooks and our authors in the next 2 weeks