Terraform was first released just a couple of years ago, and it still hasn't reached a major version. It gains more and more in popularity, it grows like crazy, actually and changes rapidly.

The book you are reading was started with Terraform 0.7.7. It was finished and updated to Terraform 0.8. Even between minor versions, from 0.7.7 up to 0.7.13, there were many small changes that made some code deprecated and some code broken. However, Terraform 0.8 introduced conditionals, as well as it introduced proper dependencies on modules, which made big chunks of code simply irrelevant now.

With ever growing number of contributors, and, as a result, the size of codebase, number of providers, and so on, it can be hard to catch up with the latest changes. Keep this in mind when starting to use Terraform: you have to be ready to deal with incompatible changes, with new features appearing and old ones going away. It is true for every open source project. It is especially true for projects that haven't reached a major version yet, and even more true for such a new and now very popular tool as Terraform. This brings us to the next point: speculating on the future of Terraform.