Molly Make-Believe by Eleanor Hallowell Abbott

Recovering from a long illness, Boston businessman Carl Stanton is unable to accompany his fiancée Cornelia on a mid-winter trip to warm and sunny Jacksonville. Lonely, bored, and disappointed in Cornelia’s lack of affection, Carl decides to answer an advertisement from the Serial-Letter Company, which promises real letters, delivering comfort and entertainment, from imaginary persons. Carl signs up for their love letter program, thinking he might have a bit of fun, and teach his fiancée a lesson in the process.