Here is a unique little blog which shows autographed baseball cards of former Major League players (or affiliated Minor Leagues), but the cards were signed during the players’ times in the independent baseball leagues:
What is interesting is that, while the blog is a fan site, there is some description about the player and how the autograph was collected.
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