The Brethren by John Grisham

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.