Baseball's official historian, John Thorn, just posted a link on SABR-L to an article he wrote about the history of the batting average statistic. He explains that Henry Chadwick used a primitive version of slugging percentage back in the 1860s that was total bases divided by number of games and how it eventually came to be realized that times at bat was a more accurate way of assessing this data and why it eventually (d)evolved into batting average.
https://ourgame.mlblogs.com/chadwicks-choice-the-origin-of-the-batting-average-e8e9e9402d53
And for those readers who do not know anything about John Thorn, he collaborated with Pete Palmer on The Hidden Game of Baseball and compiled the statistical encyclopedia, Total Baseball, with him (before the internet took over).
https://ourgame.mlblogs.com/chadwicks-choice-the-origin-of-the-batting-average-e8e9e9402d53
And for those readers who do not know anything about John Thorn, he collaborated with Pete Palmer on The Hidden Game of Baseball and compiled the statistical encyclopedia, Total Baseball, with him (before the internet took over).