Hi, I'm the author of the book. Thanks for your input :)

The book is not free, this is not a paywall, and it's not a subscription: You are buying early access to the book so that you can start reading it before I am finished writing it. As soon as the book is complete, you also get an official digital copy of the entire book (PDF, mobi, epub, etc). The book is growing quite rapidly, it's currently over 50000 words covering many topics, so far mostly related to type-level reasoning. I will be adding a table of contents soon.