As a teen, my main hobby was wargaming. Not the cash-cow that is Warhammer today, but historical wargames using plastic and metal figures painted in historically accurate uniforms. Companies such as Airfix, Hinchcliffe and Miniture Figurines (known to a generation of wargamers as MiniFigs) provided the backbone of my miniature armies.
The book that took my hobby from a kids pastime to something more organised and structured was this book by Donald F. Featherstone. In fact, I was certainly not the only one to benefit from his vision. He took this hobby and dragged it out of the realm of Wells' Little Wars and into the twentieth century. For this, I am eternally grateful.
'War Games' comes complete with three sets of rules, as well as photographs of games using these rules. Each of these rules, one for ancient battles, one for 'horse and musket' battles (which covers a multitude of eras, from early 18th century until mid 19th century), and one for modern wars (specifically World War 2 games).
It is probably dated now, with others producing rules of a higher quality and providing more realistic games, but nevertheless this is a classic volume in it's field.