Another good free book to read on Wargaming: Dunnigan's "Complete Wargames Handbook" (2000 Edition)

The classic wargames book: 

https://www.professionalwargaming.co.uk/Complete-Wargames-Handbook-Dunnigan.pdf

Courtesy of the Connections Professional Wargaming site. Still relevant today. Try looking around their other downloads, it is an interesting mix of resources: 

 ttps://www.professionalwargaming.co.uk/Downloads.html