A Who's Who (or Who Is Who) is a reference work consisting of biographical entries of notable people in a particular field. The oldest and best-known is the annual publication Who's Who, a reference work on contemporary prominent people in Britain published annually since 1849. Notable examples by country. Who's Who, the oldest listing of prominent British people since 1849; people who have died since 1897 are listed in Who Was Who. Cambridge Who's Who (also known as Worldwide Who's Who), a vanity publisher based in Uniondale, New York.
https://en.wikipedia.org/wiki/Who%27s_Who