Not exactly a book about The Who, but if there were a 5th member of the band in the early days
it would have to be Kit Lambert. Kit was one of The Who's managers and producers during their 
most productive and creative years and he is often credited with inventing the concept of a rock opera.
This book is about Kit, his father (a classical music composer), and his grandfather. Includes family tree.
Roughly a third of the book is about Kit including lots of details about his work with The Who.