Bibliography:
ITALY: Crevato-Selvaggi: Il Nuovo Gaggero - Catalogo dei bolli tondo- riquadrati del Regno d’Italia - con la collaborazione di Daniele Prudenzano - Ed. Vaccari;
BRITISH EMPIRE - Robson Lowe: The Encyclopedia of British Empire Postage Stamps;
GREAT BRITAIN: Stanley F . Cohen: Collecting British Squared Circle Postmarks.
CANADA - UniSafe: Catalogue of Canadian Stamps;
CANADA - British North America Philatelic Society : The Squared Circle Postmarks of Canada;
JAMAICA - Bill Atmore: Jamaica Squared Circle Postmarks;
JAMAICA: Thomas Foster: The squared circle postmarks of Jamaica
WORLD WIDE - Curt Nicolaus Fernau: Squared Circle Postmarks used outside Great Britain;
INDIA - http://www.rjbw.net/IndiaUA.html : The Postal History of India, Indian Stamps Used Abroad;
AUSTRALIA - Rodney Overton: A Catalogue of Squared Circle Postmarks of South Australia.
AUSTRALIA: Donald G. Cox, South Australia, the Squared Circle cancellations, the Hawthorn Press
NEW ZEALAND: Patricia E. Capill- New Zealand Squared Circle cancellations.
NEW ZEALAND: Richard Wooders: The 1998 New Zealand Post Offices cancellations and postmarks guide.
CAPE OF GOOD HOPE: Gold Blatt: The postal history of the Cape of Good Hope
NYASALAND: Banks and Roberts: Postmarks of Nyasaland and Malawi
SURINAME: ERFMANN AND STUUT- Posthistorie van Het Rijksdeel Suriname: 1650-1975.
INDONESIA: A.W.ten Geuzendam: De vierkantstempels van Nederlands-Oost-Indie 1892-1916. [This book has drawings of every hammer the author knew about.]
INDONESIA: P.Storm van Leeuwen: Poststempel Catalogus Nederlands-Indie 1864 – 1942. [This book contains some new discoveries not in the above book, but does not show every hammer. It does mention that the list includes 214 offices that used the squared circle.]
CURACAO: Frank W. Julsen and A. M. Benders (1976). A Postal History of Curacao.
CURACAO: Curacao: Richard A. Phelps: An Identification Guide for Cancels (2009)
NIGERIA: Jack Ince and John Sacher: The Postal Services of the British Nigeria Region.
NIGERIA: Mr E Proud : The Postal History of Nigeria
BRITISH EAST AFRICA: John Minns (UK) : BEA handbook