Multiple Book Versions I imagine this has been asked before, I'm just struggling to find the right answer.

Using Books to create documentation for an open source project. We're covering 3 domains, Users, Developers, and Admins. Easy enough.

Where I am struggling is how to create versions of these books for each software release. Do I just create a new book for every version? 1.0, 1.1, 1.2, etc? Should I be using a taxonomy vocab for the releases? What is a manageable way to handle this?
Documentation can be done in many different ways using different modules. If you decided to go with Books, then it is really up to you to create new books or taxonomies for new releases. Look at how Drupal's documentations is organized on https://www.drupal.org/documentation.