Alan Douglas is the well known author of the popular 3 book series 

"Radio Manufacturers of the 1920's " 

Volume1 - ISBN 1-886606-02-1

Volume2 - ISBN 1-886606-00-5

Volume3 - ISBN 1-886606-04-8


"Tube testers and Classic Electronic Test gear".  ISBN 1-886606-14-5

All four of these books are published by Sonoran Publishing LLC, and contain a wealth of historic and technical context for the antique electronic gear described.