Alongside the release of GTSAM 2.1.0 (see below) is a new hands-on tutorial for learning about factor graphs and GTSAM.  This tutorial will help you to quickly get started writing your own software using GTSAM as a factor graph optimization backend.  Also, it provides a very accessible introduction, to the mathematics and applications of factor graphs.  The tutorial covers probability functions represented by factor graphs and their optimization, a number of real-world mapping examples with source code, and how to easily have GTSAM optimize your own custom factors.

Factor Graphs and GTSAM: A Hands-on Introduction, Frank Dellaert, Technical Report GT-RIM-CP&R-2012-002.