'[the book] emphasizes the essential physics behind the mathematical results. … the authors … give a flavour of all [the] problems without getting into all the mathematical complexities of a full treatment. … in each case [they] give a physical discussion of the problem, to explain what result one expects to emerge from the mathematics … Most chapters contain references for further in-depth reading on the topics outlined in the text, and some more detailed material is also present in the end-of-chapter problems … this is a very useful book for new graduate students and it also gives new insights to those of us with more experience; I wish it had been available when I was learning the subject.' The Observatory