I'm happy to tell that the booklet HTTP/3 Explained is now ready for the world. It is entirely free and open and is available in several different formats to fit your reading habits. (It is not available on dead trees.)
The book describes what HTTP/3 and its underlying transport protocol QUIC are, why they exist, what features they have and how they work. The book is meant to be readable and understandable for most people with a rudimentary level of network knowledge or better.
These protocols are not done yet, there aren't even any implementation of these protocols in the main browsers yet! The book will be updated and extended along the way when things change, implementations mature and the protocols settle.
If you find bugs, mistakes, something that needs to be explained better/deeper or otherwise want to help out with the contents, file a bug!
It was just a short while ago I mentioned the decision to change the name of the protocol to HTTP/3. That triggered me to refresh my document in progress and there are now over 8,000 words there to help.
The entire HTTP/3 Explained contents are available on github.
If you haven't caught up with HTTP/2 quite yet, don't worry. We have you covered for that as well, with the http2 explained book.