If you're already familiar with the original Btrieve API and want to understand how your current knowledge maps to the Btrieve 2 API then the Btrieve Operation to Btrieve C++ and C API Mapping Guide guide is a good place to start.
If you want an introduction to the Btrieve C++ API paradigm then the simple example, btest.cpp, is a good place to start. There are also examples for the Btrieve C API paradigm, btest.c, and the Btrieve Python API paradigm, btest.py. See examples for a complete list of available examples.
If you want to explore the Btrieve C++ API via its documentation then the class list is a good place to start.
Btrieve supports a variety of programming languages both directly and via the use of SWIG (http://www.swig.org).