Paul Erdös (pronounced "air-dish") was one of the great mathematicians of the 20th century, prolifically publishing deep results about number theory, and collaborating widely.
He was also seriously weird. He travelled the globe constantly, carrying his worldly possessions in just two suitcases, travelling from mathematician to mathematician, turning up on their doorsteps, usually unannounced, declaring "my brain is open" -- an invitation for them to collaborate with, and house, him for a few weeks. He spoke in a strange sort of code -- referring to children as "epsilons", to music as "noise", and to God as the "Supreme Fascist" or "SF", who possesses "The Book" that holds all the beautiful mathematical proofs.
Hoffman followed Erdös around for a while, chronicling his eccentric lifestyle. This book paints a fascinating impressionistic portrait of a genius whose whole life was dominated by the study of number theory. There is also a fair smattering of other mathematics, from Russell's barber, Hardy and Ramanujan, Gödel's incompleteness results, to Ramsey theory and Graham's number, that helps put Erdös' work in context. But much of that stuff is well covered in other places -- I would have liked to have had more about Erdös' own work.
I am impressed by the patience of those many visited mathematicians -- housing Erdös was not easy. He worked 19 hours a day, kept going with coffee and Benzedrine, and expected his hosts to keep up with his pace. However, it seems from this account that the intellectual reward of working with Erdös more than compensated for having their lives turned upside down for a few weeks. I am glad I live in a world where Erdös could live that kind of life if he wanted to (even though I suspect a non-genius would have been swiftly institutionalised).