Every writer can benefit from learning how to boil down arcane subjects into informed, accessible summaries. In the early 2000s, I wrote the entry on the trial by ordeal for Medieval Folklore: An Encyclopedia of Myths, Legends, Tales, Beliefs, and Customs, and I contributed the entries for "population" and "Peasants' Revolt of 1381" to the Historical Dictionary of Late Medieval England, 1272–1485.
In 2006 I had the truly odd distinction of writing the entry on philately for the J.R.R. Tolkien Encyclopedia. A blogger who reviewed the entire book one entry at a time gave my contribution a positive but bemused review: "I have to say, I'm bowled over by the very concept of this article." I felt much the same way.