I assume I'm not the only one who didn't know what a "muffin" was. At the Baseball Hall of Fame I found this explanation of the book: "Muffin was a baseball term used to describe the least skilled members of an amateur baseball team. These players tended to be less familiar with the rules of the game and to make more errors than the 'first nine' or more skilled players. The book illustrates the comic antics of a group of bumbling 'muffins' on the field."
Link to the book: Baseball As Viewed by a Muffin
What a great look back into the infancy of sports in USA. A book from 1867, before The USA's first organised sports league was started in 1871.
The definition of a "muffin" above is only partial. It also referred to the deficient player being somewhat "effeminate", thereby, not only less skilled, but less athletic, less coordinated, and less muscular. Back in the 1850s through the 1880s, baseball play (along with boxing) was dominated by Irish immigrants, who were, on the whole, less educated, and more brawny than the average young American man due to such a high proportion of their numbers having jobs involved in heavy labour. In addition, they were more rowdy in their behaviour than average.
It's very interesting how different "The US National Pastime" was in those early days of the sport, compared to how it is today. Notice that they gave one of the "muffin" batters a wide headed Cricket bat, because, ostensibly, he couldn't even make contact with the ball with his normal rounded baseball bat.