He is best known under his pseudonym of PL Jacob, bibliophile, or "Bibliophile Jacob," suggested by the constant interest he took in public libraries and books generally. Lacroix was an extremely prolific and varied writer. Over twenty historical romances alone came from his pen, and he also wrote a variety of serious historical works, including a history of Napoleon III, and the life and times of the Tsar Nicholas I of Russia.
He was the joint author with Ferdinand Séré of a five-volume work, Le Moyen Âge et La Renaissance (1847), a standard work on the manners, customs and dress of those times, the chief merit of which lies in the great number of illustrations it contains. He also wrote many monographs on phases of the history of culture. Over the signature Pierre Dufour was published an exhaustive Histoire de la Prostitution (1851-1852), which has always been attributed to Lacroix. His works on bibliography were also extremely numerous. In 1885 he was appointed librarian of the Arsenal Library, Paris.
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