Collaboration and CI/CD
Since Terraform 0.8 it has a built-in support for Vault, a secrets management tool from HashiCorp. It's also a big tool, deserving a book of its own. And it's a great solution in order to solve the sensitive data storage problem for Terraform. Consider learning it and using it.
In Chapter 7, Collaborative Infrastructure, we did not go too deep into details of modern software development workflow. The basics we discussed--code reviews, working through pull requests--are just this: basics. There is a number of well-documented workflows that cover many different situations:
GitHub Flow (https://guides.github.com/introduction/flow/): Very simplistic and popular approach
GitLab Flow (https://docs.gitlab.com/ee/workflow/gitlab_flow.html): A slightly different process from the GitLab team
Git Flow (http://nvie.com/posts/a-successful-git-branching-model/): Most complex and extremely popular model of development as well
All of them are based on Git though you can achieve similar...