Espionage leaks - in a children's book

Not Edward Snowden this time, but rather Usborne's "The KnowHow Book of Spycraft". I loved it when I was a child but I did not know that, as the great Waterstones blog tells us, "Far more influential than the average children’s book, it was even submitted as evidence at the high court trial of Soviet spy Oleg Gordievsky, who testified it gave away the KGB’s tradecraft".