Troubleshooting
Published book does not show updates
Not sure how long it takes a push to github.com to show up on github.io as pages...but it may take a couple of minutes.
Pages are not generated
Every page in the book needs to be in the SUMMARY.md in order to be built.
Can't find crate for somecrate
In Rust Playground https://play.rust-lang.org only the current top 100 crates are available to be used, so you cannot arbitrarily add code and have it run inside your book on the web page.
Code block will not run in browser
For example, the Hello World will not run when you click the play button.
After a reboot, this problem magically went away. Maybe it is a browser cache issue.