Decision Forests for Computer Vision and Medical Image Analysis book from Microsoft Research

This is the best resource I have found for understanding and using decision tree based machine learning algorithms.  It is very thorough on both theory and practical use with comparisons with other algorithms such as SVM's and AdaBoost.

There are also a bunch of supplemental materials available for free including a very nice PowerPoint with great explanations.  The supplemental materials include C++ and C# code.

Decision Forests - Microsoft Research