All of the of the SteamVR Tracking HDK documentation comes as part of the package distributed through Steam.
The first step is to sign the SteamVR license agreement (this is a free license with no royalty). After signing the agreement you can download the HDK from inside your Steam account.
After doing this, you can find all of the documentation on your PC in the following folder:
C:\Program Files (x86)\Steam\steamapps\common\SteamVR Tracking HDK\docs\
There is also a lot of information in presentation format:
C:\Program Files (x86)\Steam\steamapps\common\SteamVR Tracking HDK\training\presentations