Where can I find J2534 documentation?

The standard can be purchased from the SAE for $59. The PDF document is copyrighted and DRM protected and thus cannot be distributed.
The Tactrix J2534 downloads section includes C++ sample code with the Openport driver / J2534 installer.
Drew Technologies has a some good resources:
- About PassThru
- API documentation
- Drew Technologies J2534-1 Tool - allows you to connect to J2534-1 devices such as the OpenPort 2.0 and experiment with the API. This program was written for DrewTech products, and so some J2534 features are disabled in the program even though they are supported with the OpenPort 2.0.
- Sample code for DLL discovery