I haven't got an answer for you but I hate seeing thoughtful posts go unanswered too.
I must admit, I've never read a Biochar book. What I have consumed a lot of is scientific studies so I can glean how I can use it in my everyday life. There are so many out there, one wouldn't really run out of things to read.
Then, I might read a blog or two, and I'm sure there are videos and now podcasts with the same. When i was obsessing over biochar water treatment, this blog was good info: https://joshkearns.substack.com/