I have just bought "The Voice Doctor - The Story of Singing" by Professor Arnold Maran (ISBN 1-85776-915-5) and found it to be an excellent read.  I recommend it for every professional speaker who has an interest in what lies behind their craft.  You'll find really interesting chapters on how the voice is produced and why women's voices sound different to men's.  It's one of the few places where you'll find an excellent explanation as to the all important linkage bewtween the human voice and the human ear (apart from attending one of my courses, of course!).  There's also some top notch information of how to look after your voice - clearly very important for singers on tour, but also for professional speakers.

A top notch read.