Yup, 80%. They say that explicitly here: https://leanpub.com/authors. The site itself, LeanPub, is one of the more (if not most) well-known self-publishing outlets, particularly recognizable for supporting "early access" books. I.e. like video games, you can pay while the book is still being written, to access its already made parts and possibly influence its final shape. People sometimes use it for donations too, by setting the minimum price to $0.
The quality is what you'd expect. On a spectrum between a blogpost and an O'Reilly book, most what I've seen is around 30% there.
Other books related to Lisp popped up there too. I remember buying three:
- https://leanpub.com/lispwebtales
- https://leanpub.com/lisphackers
(EDIT: no affiliation with LeanPub; just the way you wrote that sentence made me think you believe parts of this platform are features of the zine, which they're not.)