The Collected Works of Gretchen Oyster

Who is Gretchen Oyster? The discovery of a series of mysterious handmade postcards distracts Hartley from trouble at home. A poignant novel for fans of Rebecca Stead and Holly Goldberg Sloan. Hartley Staples, near-graduate of middle school, is grappling with the fact that his older brother has run away from home, when he finds a handmade postcard that fascinates him. And soon he spots another. Despite his losing interest in pretty much everything since Jackson ran away, Hartley finds himself searching for cards in his small town at every opportunity, ignoring other responsibilities, namely choosing a topic for his final project.
Who is G. O. and why are they scattering cards about the town?