Make Your Harmonica Work Better by Douglas Tate.
There is a vast amount of knowledge throughout the world about how to make the harmonica sound better and play better, but little of it is written down. This fully illustrated book solves the problem. It contains sections on: how to buy a harmonica, common problems, advice about tools and techniques for using them, the basic parts of the chromatic harmonica, solutions to improve sound and functionality, experimenting on old harmonicas, tuning, and much more.
For chromatic players, but diatonic players will learn just as much.