I had this stashed in my bookmarks for a while and figured it was time to share it. What we have here is the 1884 Wright & Ditson Base Ball Guide and what is unique about it is that it focuses on the Union Association, in the only year of the league's existence. It gives you a lot of information about the league, its teams, players, schedule, etc., that you can't find anywhere else.
The other great thing about the guide are the ads for baseball equipment that you find in the back. I love 19th century ads and these are great. What's really interesting is that you get prices for things like balls, bats, uniforms, and all the things you would need to play the game. Good stuff.