It may be Book 29 of Time-Life's World War II series, but The Secret War was the volume they advertised when I was a kid and that made me point at the television with enough interest that my mom got me a subscription. It came first, followed by others in random order until my LACK of interest made her cancel it. I was really in it for pictures of spy gear, and have probably just read it cover to cover now, some 40 years later. It's a fun volume, though you could also say it's a bit of a hodgepodge as well. There are stories of spies, disinformation campaigns, elements of the arms race, and code-breaking (shockingly, but because if was published in '81, absolutely no mention of Alan Turing in relation to the latter). Some of the stories expand (or repeat) stories that were also important to other books in the series (mostly The Resistance and The Second Front, possibly others, I don't have them all). But despite the repetition, this is one of the better and least technical reads in the series, on a topic that should interest fans of spycraft, not just of military history.