In 1940-41, it looked like Hitler might invade Britain. Tom Wintringham, the author of this little book, was instrumental in forming the Home Guard, as he'd been a Communist volunteer in the Spanish Civil War. In NEW WAYS OF WAR, he gives some information about fighting and fortification that would be of use to British resistance fighters.
Unfortunately, because Wintringham was a commie, he spent far more time bashing Britain for the sin of being capitalist than in outlining how to resist invaders. So while some of the pro-communist propaganda and Spanish Civil War stuff is historically interesting, the book is useful more for what it tells us about Britain in 1940 than it is for showing civilians how to fight.