During the 1960's, a number of books were published about the group. The ones I have been able to track down, I have made available online in PDF format.
You will need Adobe Acrobat Reader to view these files:
Click on the above link for a series of articles written by Joy in the above book published by Hodder & Stoughton in 1969, 267KB
Another book featuring a lot of quotes from the group published in 1967, 154KB
Rhythm Finds It's Groove
A edited PDF file on the chapter on the group from Brindley Boons: "Sing The Happy Song"
These books have been scanned in using OCR Software & then checked for spelling mistakes, I apologise for any errors that have bee overlooked.