Clustering and tree models are widely used in the social and biological sciences to analyze similarity relations. Tree Models of Similarity and Association describes how matrices of similarities or associations among entities can be modeled using trees, and to explain some of the issues that arise in performing such analyses and correctly interpreting the results. James E. Corter clearly distinguishes ultrametric trees (fit by the techniques widely known as "hierarchical clustering") from additive trees and discusses how specific aspects of each type of tree can be interpreted through the use of applications as examples. He concludes with a discussion of when tree models might be preferable to spatial geometric models, such as those fit by multidimensional scaling (MDS) or principal components analysis (PCA).