The breathren by john grisham

Trumble is a minimum security federal prison, home to drug dealers, bank robbers, swindlers, embezzlers, tax evaders, and three former judges who call themselves The Brethren. They meet each day in the law library where they spend hours writing letters. They are fine-tuning a mail scam, and it's starting to pay big. The money is pouring in.

But then their little scam goes awry. It ensnares the wrong victim, a powerful man on the outside, a man with dangerous friends, and The Brethren's days of quietly marking time are over.