THE BRETHREN Trumble is a minimum-security federal prison, a "camp," home to the usual assortment of relatively harmless criminals--drug dealers, bank robbers, swindlers, embezzlers, tax evaders, two Wall Street crooks, one doctor, at least five lawyers.And three former judges who call themselves the Brethren: one from Texas, one from California, and one from Mississippi. They meet each day in the law library, their turf at Trumble, where they write briefs, handle cases for other inmates, practice law without a license, and sometimes dispense jailhouse justice. And they spend hours writing letters. They are fine-tuning a mail scam, and it's starting to really work. The money is pouring in.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.