Mike, from New York Baseball Digest, recently wrote a short but concise article about the history of the Northern League, one of the first two independent baseball leagues which started in 1993:
He also gives credit to the first real independent baseball league in recent memory which was the Empire State League. That league played in 1987 and for a few weeks in 1998 in the Long Island/Staten Island New York area.
Mike covers the merger of the Northern League and Northeast League as well as teams which joined and left the NL after the merger ended in the early 2000’s.
There are several good books about the independent professional baseball leagues. You are welcome to see a list of suggestions through this link: Books Get notified of upcoming independent baseball tryouts from either of these services: first service or the second service. Prospective players can find some helpful resources here We will be providing a growing list of possibly helpful tips and suggestions on various baseball information. This will range from "how to" information about baseball skills all the way to helpful information for casual baseball fans. Please click the link for more information: Tips And How To Information