"What's it all for?", Marcus du Sautoy was asked by a promising student who had decided to abandon higher mathematics to work in the City. This, and his impending 40th birthday, plunged him into a mild existential crisis. Why had he spent his life studying group theory and the problem of symmetry? Finding Moonshine answers that question. He once revelled in the unworldly nature of his subject, but now feels impelled to emphasise its relevance to our understanding of nature. The "non-mathematically sensitive" should probably avoid his discussion of the Monster (a huge symmetrical object first constructed in 1980) and moonshine (a modular function) - especially his mindbending observation that you can "see the moonshine glinting on the Monster" - and instead head for discussions of maths in music (Bach, Mozart, Xenakis). Maths is a "tribal" subject, Du Sautoy writes, and we are introduced to some eccentric members, one of whom hates being touched and mumbles "ooze" under his breath until he has cracked a problem.