Ned Mouse Breaks Away — Tim Wynne-Jones

Ned Mouse has been sent to jail. His crime? Writing "The government is unfair to mice!" in his spinach. But Ned does not like jail. He does not like his tiny cell. He does not like the food. And he does not look good in stripes.
"I would give my right arm to be out of here," he thinks. Thus begin his attempts to make a break for freedom. He tries digging a tunnel. He tries building an escape plane in shop class. He tries fleeing down the drain hole in the tub. Yet each time his attempts are foiled by the keeperâ€" the big gentle guard who has the brains of a tree but who is devoted to
his job, which is to keep prisoners in their cells.
The years pass, and Ned very nearly gives up. Then one day he receives a letter from his long-lost friend Mort, who tempts him with stories of the beautiful sunrises at his house by the sea.
Filled with new determination, Ned comes up with the most daring escape plan yet. He'll mail himself to Mort, piece by piece. And all he needs for his plan to succeed is the cooperation of the gullible keeper.