The Rough Plot:

A boy inexplicably starts shrinking.  When he is about 6 inches tall he befriends a Seagull, who takes him for a flight.  The boy’s father is an inventor and makes a cockpit that fits on the Seagull’s back.  A mysterious person from the Government comes to visit and asks the boy to ride his Seagull around the world and deposit a bomb-pill down the chimney of an evil person’s factory to destroy the bomb he is making and save the world.  He does it and then is blown far away by the resulting explosion, but the Seagull eventually returns him home to re-unite with his Father.

This book was well illustrated with realistic pen and ink drawings.

It was a Hardcover book.  I probably read it between 1948 and 1954.