The Surface Printed KGV 1d Sideface Issue Variety Catalogue by G.R. Monk, 2006, ISBN 0 646 2883.

A wonderful publication that makes it so much easier to study the 1d King George V definitive as it list varieties in where they appear on the stamp, making it so much easier to find them. The ideal pictorial index to the George V One Penny, 3rd Edition  by the BSAP.