This book is an accessible introduction to the theory and practice of ensemble methods in machine learning. It is a quick read, has sufficient detail for a novice to begin experimenting, and copious references for those who are interested in digging deeper. The authors also provide a nice discussion of cross-validation, and their section on regularization techniques is much more straightforward, in my opinion, than the equivalent sections in The Elements of Statistical Learning (Elements is a wonderful, necessary book, but a hard read).
The heart of the text is the chapter on Importance Sampling. The authors frame the classic ensemble methods (bagging, boosting, and random forests) as special cases of the Importance Sampling methodology. This not only clarifies the explanations of each approach, but also provides a principled basis for finding improvements to the original algorithms. They have one of the clearest descriptions of AdaBoost that I've ever read.
The penultimate chapter is on "Rule Ensembles": an attempt at a more interpretable ensemble learner. They also discuss measures for variable importance and interaction strength. The last chapter discusses Generalized Degrees of Freedom as an alternative complexity measure; it is probably of more interest to researchers and mathematicians than to practitioners.
Overall, I found the book clear and concise, with good attention to practical details. I appreciated the snippets of R code and the references to relevant R packages. One minor nitpick: this book has also been published digitally, presumably with color figures. Because the print version is grayscale, some of the color-coded graphs are now illegible. Usually the major points of the figure are clear from the context in the text; still, the color to grayscale conversion is something for future authors in this series to keep in mind.
Recommended.