I'll blow my own horn and add:
The Modern Hobby Guide To Topps Chewing Gum: 1938 To 1956 by David Hornish
It covers baseball, other sports and non-sports plus has a history of Topps and its founders. Free color version available (you can order the book too but it's B&W only), with a link to my much neglected blog with updates:
http://themodernhobbyguide.blogspot.com/