Gibbs, R. M., G. R. I. - The Postage Stamps of the German Colonies Occupied by the britisch 1914-1918, 1987, the standard work for the GRI overprints, contains a detailed cataloging of the stamps and postal stationery with plate flaws, varieties and so on and besides background information, the cancel, with rarity information, numerous illustrations, partly also coloured, 275 sides, linen letter with dust cover.