This is an in-depth handbook, reference manual, and general guide for using and configuring input devices via the Steam Input software. This includes the Steam Controller hardware itself as well as a variety of 3rd-party devices.
This is not a detailed listing of methods used for interacting with the the software from within your game. For that please see the Steam Input API.
To set the stage, you might start by watching or reading this Steam Dev Days 2016 presentation: