For a a quick introduction see the tutorial on Factor Graphs and GTSAM: A Hands-on Introduction.
A more thorough introduction to the use of factor graphs in robotics is the 2017 article Factor graphs for robot perception by Frank Dellaert and Michael Kaess.
API and Wrapper Documentation
Currently, detailed API documentation is available only for C++ via the C++ Doxygen generated site.
GTSAM comes with a python wrapper (see cython directory) and a matlab wrapper (see matlab directory), and for prototyping with GTSAM we highly recommend using one of the above. The auto-generated API documentation for python/MATLAB is limited to the number and type of input arguments, and again the doxygen docs provide the details.
Notes on GTSAM
There is a
GTSAM users Google group for general discussion.