The Encyclopaedia of South African Post Offices and Postal Agencies Volumes 1 to 4 by Ralph F. Putzel. Also The Postmarks of South Africa and the Former States and Colonies Volumes1 to 10 by Ralph F. Putzel. The Postmark and Postal History Society of Southern Africa - A Historical overview 1969 to 2000 by Neville Gomm. The Airposts of South Africa by L.A. Wyndham. South African Postmarks by Dr. T.B. Berry. The Special and Commemorative Postmarks, Cachets and Covers of South Africa 1892 to 1975 by Hasso. O. Reisner. South African Postal Slogans; The Barred Oval Numeral Canceller of The Cape of Good Hope of 1864 by Franco Frescura, South African Postal Slogans by Jan Lessing. Some volumes have covers interleaved within them