This is a handbook targeting people interested or involved in Ruby on Rails development that doesn't want to think hard about dependencies often times.
There are RoR developers who work in one project for long terms and might not need to be setting up development environments frequently, however, there are also many others who work in several small RoR projects from time to time.
Backend Handbook is for those prefer to have isolated configurations for their Rails apps using tools like VirtualBox, Vagrant or even Docker. The content of this guide also works for people working all their projects in the same machine but would probably have less value.
This is a collection of summaries of instructions on how to install X or Y in order to get your app running. You can actually google them but here you'll find the meat, straightforward commands to get you doing what you want and not what you need.