need isbn for better distribution

[eshellman]

Unglue.it (https://unglue.it/) would like to contribute an ISBN for this book. Among other things, this will help make it possible for libraries to add it to their collections. If this is acceptable to the repo maintainers, I will generate a pull request.

[omerk]

@eshellman Certainly, please go ahead and I'll merge your PR.