To my amazement, very little has changed from the official baseball rules from 1897 and those of today. Granted, today's version is more fine-tuned and addresses specific situations that have occurred over the last 125 years, but the fundamental rules of the game have remained very static. In examining the 1898 Spalding Guide to baseball, I found just a couple of rules listed that vary from those of Major League Baseball today.
Most are relatively minor differences.
Rule 14.2 levied a $5 fine for purposefully dirtying the ball, if the opposing team's captain protested the matter.
Rule 15 concerning the baseball bat said that the bat could be no bigger than 2 3/4 inches in diameter. Today, the specification is 2.61 inches.
Rule 19.2 said that a catcher's or first baseman's mitt could be of any size, shape, or weight! The infielder's gloves needed to weigh no more than 10 ounces and have a maximum circumference of 14 inches around the palm.
Today's limits, if you are curious:
Catcher's mitt: Limit of 38 inches diameter and 15 1/2 inches long.
First baseman's mitt: Limit of 12 inches long and 8 inches wide.
Infielders: Limit of 12 inches long and 7 3/4 inches wide (no weight limit, unlike 1897)
Forty years before night baseball, Rule 21 stated that all championship games must start no later than two hours before sunset. (With the lack of mound visits, pitcher and batter substitutions, in-game promotions, TV commercials, and the like, major league games rarely took more than two hours in 1897.)
Rule 27.3 from the 1898 edition said that any substitution for a base-runner must be approved by the captains of both teams.
Rule 35 said that a fair ball touched by a spectator would only be considered a dead ball if the spectator retained possession of the ball. Today, any fair ball touched by a spectator becomes a dead ball.
Rule 45 said that a strike would be considered any fairly delivered ball. This was defined as a ball that went over the plate, and was no higher than the batter's shoulders and no lower than his knees. Today's rule officially defines the top of the strike zone as the midpoint between a players shoulders and his belt, and the bottom of the strike zone remains the knees.
Rule 69 said that no betting was allowed on a club's field or on any property owned or occupied by the club. This rule is not explicit in today's code, but betting on baseball games by participants is widely known to be prohibited.