During World War I, Irving withdrew from the theatre and returned to the law, writing the study for which he is now most famous, "A Book of Remarkable Criminals", originally published in 1918, which examined the lives, motivations and crimes of some infamous murderers, "Life of Judge Jeffreys", "French Criminals of the 19th Century" and other papers on the subject. After spending twenty years of his life dedicated to the theatre, his greatest success came from being what it was intended he should be, a legal expert.