The 3ds Max SDK Programmer's Guide describes how to use the 3ds Max Software Development Kit (SDK) to extend the features, functionality, and interface of 3ds Max primarily using C++. For extending 3ds Max with the MAXScript scripting language see the MAXScript documentation included with the 3ds Max documentation.
For information on recent changes to the 3ds Max SDK and the documentation see Whats New 3ds Max 2012.