Monsieur Lecoq, Volume I by Emile Gaboriau

The governor of the Depot, a functionary who had gained the reputation of an oracle by twenty years' experience in prisons and with prisoners -- a man whom it was most difficult to deceive -- had advised the magistrate to surround himself with every precaution before examining the prisoner, May.

And yet this man, characterized as a most dangerous criminal, and the very announcement of whose coming had made the clerk turn pale, had proved to be a practical, harmless, and jovial philosopher, vain of his eloquence, a bohemian whose existence depended upon his ability to turn a compliment; in short, a somewhat erratic genius.