Leave it to PSmith by P. G. Wodehouse (1881 - 1975)

Freddie Threepwood and his uncle are in difficulties. Freddie wants a thousand pounds to start a bookmaker’s business and to marry Eve, while his uncle wants to raise three thousand pounds, unbeknown to his wife, to help a runaway daughter. Freddie persuades his uncle to steal his wife’s necklace and sees Psmith’s advertisement in a daily paper. Freddie enlists the services of Psmith to steal the necklace. There are plots and counterplots. This is the fourth book in the "PSmith" series, following on from "Mike and PSmith", "PSmith in the City" and "PSmith, Journalist". (From the publisher's blurb)