The Return of the Great Brain, by John D. Fitzgerald, was published in 1974 by Dial.
Tom Fitzgerald, alias The Great Brain, is back, struggling to stay reformed now that his friends have threatened to ostracize him if he pulls even one more swindle. But his brother J.D. knows the new Tom is too good to be true, and as a reformed Great Brain makes for a dull life, J.D. isn’t exactly unhappy—or blameless—when his brother’s money-loving heart stealthily returns to business as usual. Under the watchful eyes of parents and friends, Tom has to be craftier than ever, and indeed he is. Whether he’s cleverly pulling an out-and-out swindle so as to not be caught or solving a train robbery and murder, Tom’s Great Brain never fails.