Primarily, this was an experiment in 'selling your byproducts'.[1] I'm already self-publishing a book on Hypermedia APIs[2], and the older system it was in couldn't handle my new requirements. I also wanted to learn Rust, and so I decided to just write it all down as I went. It also enabled me to get familiar enough with my new tooling that when it came time to port over DHAs, it wasn't bad at all.
So yes, doing the same thing 'for other languages' would be fun, and if Rust continues to take off, I might give it a shot. I'm already planning on writing more content for the book as I go; I've been working on a Rust buildpack for Heroku...
Then put out the same book with a new title, "Rust for Pythonistas". Two books for the price of one!