The history, ships and postal cancellations of the Woermann Steamship Line of Hamburg (Deutsche Seepost-Linie Hamburg Westafrika) also list of ships and cancellations of the Deutsche Ost Afrika Line (with which the Woermann Line amalgamated) 1890-1965 /

By Cockrill, Philip. Bath, England P. Cockrill 1966

United States sea post cancellations (Part 1 and Part 2)

By Cockrill, Philip. Newbury, Berkshire, England P. Cockrill, 1988