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
and
"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.