Soviet Clandestine Mail Surveillance 1917-1991 Vol I by David Skipton and Steve Volis(2016)
This groundbreaking work identified handstamps and markings believed to indicate Soviet surveillance of the individual receiving or sending the mail. It presented empirical evidence of the surveillance and attempted to “shake the tree,” and encourage others to look for further information.
This book is also available on this web site in hardbound form.