First rules book of baseball

The first published rules of modern baseball were written in 1845 for a Manhattan team called the 'Knickerbockers'. The author of those rules, Alexander Cartwright, has come to be known as the father of modern baseball. The evolution of the so-called "Knickerbocker Rules" into the rules of today is fairly well documented.

Earliest known reference to baseball

In 2004, historian John Thom discovered a reference to a bylaw prohibiting anyone from playing "baseball" within 80 yards of the new meeting house in Pittsfield, Massachusetts. A librarian found the actual by-law in the Berkshire Athenaeum library, and its age was verified by researchers at the Williamstown Art Conservation Center.
If authentic and if actually referring to a recognizable version of the modern game, the 1791 document, would be, as of 2004, the earliest known reference to the game.

Before Baseball

Nearly every civilization known has had a stick and ball game of some kind. The ancient Greeks and Romans had several. But exactly how those games relate to the modern game is unknown.
A number of early folk games in England had characteristics that can be seen in modern baseball (as well as in cricket and rounders). Many of these early games involved a ball that was thrown at a target while an opposing player defended the target by attempting to hit the ball away. If the batter successfully hit the ball, he could attempt to score points by running between bases while fielders would attempt to catch or retrieve the ball and put the runner out in some way.
Since they were folk games, the early games had no 'official' rules, and they tended to change over time. To the extent that there were rules, they were generally simple and were not written down. There were many local variations, and varied names.
Many of the early games were not well documented, first, because they were generally peasant games (and perhaps children's games, as well); and second, because they were often discouraged, and sometimes even prohibited, either by the church or by the state, or both.
Aside from obvious differences in terminology, the games differed in the equipment used (ball, bat, club, target, etc., which were usually just whatever was available), the way in which the ball is thrown, the method of scoring, the method of making outs, the layout of the field and the number of players involved.