E, The Story of a Number by Eli Maor
This is another maths book aimed at a fairly non-technical level, but which contains a lot of interesting anecdotes and stories about the people involved. It traces the story of e, starting out with Napier’s logarithms, passing through the discovery of calculus and onto the relatively recent proofs that pi and e are transcendental. Along the way, there are many interesting side stories, such as the problems of the catenary and the cycloid, the connection between e and pi, Euler’s various formulae and an interesting chapter on the mappings of the complex plane. A thoroughly enjoyable read.