The Pleasures of Counting by T.W. Körner, ISBN 0-52-156823-4 .
An excellent book for those not afraid of a little mathematics. Very readable, and uses side-notes rather than footnotes - very effective at helping to CreateFlow. A written example of how LinksCreateFlow rather than LinksDestroyFlow.
His books on Fourier Analysis are quite entertaining too, though much more technical. They discuss not only the mathematics of the subject but the history too, including some unexpected connections with statistics, number theory, and commerce.