L. Sprague deCamp

After an unintentionally successful demon-summoning, accountant Prosper Nash finds himself on the astral plane, inhabiting the body of Jean-Prospere, Chevalier de Néche-the swashbuckling cavalier he likes to imagine himself as-and in a New York filled with characters from similar wish-fulfillment daydreams of other mundane souls. The demon is possessing his body on a mundane plane, and he attempts to find his way back.
This involves the Shamir, the Solomon's Stone of the title, and plentiful swashbuckling adventure, and a plot in which Prosper Nash's accounting abilities prove as useful as Chevalier de Néche's athletic ones.