The Man Who Loved Only Numbers is one of very few library books I've ever failed to return on time. There's a certain poetry to it: it's a biography of Paul Erdős, one of the 20th century's more important mathematicians, and certainly the most prolific. Yesterday would have been his 100th birthday.
While the book is ostensibly about Erdős, it's as much about the people he worked with (those with the highly-coveted Erdős number 1) and the maths they worked on. Hoffman, while not a mathematician himself, does a brilliant job of explaining the maths and its importance in a clear but precise way - there are many maths books out there that make me go "THAT'S NOT RIGHT" and fling them aside in disgust, but The Man Who Loved Only Numbers isn't one of them.
Hoffman gives a portrait of an odd, driven genius with very little common sense - who was, all the same, a deeply caring and highly principled man.
Also recommended: Samuel Hanson's Relatively Prime episode on Erdős.