"Some thoughts of D.H. Oakley" on Sweden Facit 612-13 discusses printing variations he has found on these two stamps. He also asks about the red single digit back numbers found on these stamps and the editor suggests they are counting numbers. Contact V6 #12 March 1971 pages 182-183.
"Dag Hammarskjold-First Citizen of the World" by M. Gordon. Biographical article in Contact, March 1963, Vol 4#4, page 49.
FDC Cards: Since about 1940 official Swedish FDC have included a printed card with information about the techical details of the stamp, and a short exposition about the subject of the design. Usually multi-lingual, and almost always including English.
"Sweden's First Miniature Sheet" by A. Pinnell describes the Agueli issue of 1969 & gives some facts about his career. Contact V6 #9 June 1970. Pages 133-134.
Wasa and Sagor booklets issued with thin paper and shiny gum c Feb 1970. PFA does not announce new variety. Contact V6 #12 March 1971 page 189
"The Bulletin" published by Sweden Post contains similar but different information in English.
Year sets from Sweden Post are another source in English. "You pay only for the stamps, the folder is free" is their slogan.
"Year Book 94/95 Swedish Stamps Narrate" is a 45 page tri-lingual coffee-table book (Swedish, English, German) which includes mint copies of the stamps, in a lavishly illustrated exposition of the subject matter with spaces to mount the stamps. Similar books were published as far back as 91/92.