I get a kick out of title's that repeat over time and demonstrate the continuum of American magazines. "Who's Who" is a very popular recurring theme. The earliest reference to the term I can find is from a British publication in 1849. I just bought the 1964 magazine on ebay for a little over $20, to go along with my 1915 and 1931 versions.
There were quite a few others, though I suspect the 1915 version was the first of the bunch.
Here's a few non first issues currently for sale on ebay.
It really is amazing how much information you can get today with a PC and a mouse!