Among those absent

Blurb from the book:
It all started when Tommy Hambledon was asked by the Home Secretary to become an inmate of a prison. Seems prisoners were disappearing with unnerving regularity and Tommy, identity unknown even to the local prison authorities, set to work to find the leak. Tommy wasn't exactly en rapport with prison life, and when he got a chance to substitute for a prisoner who was to be spirited away, he was delighted. When the break came, however, Tommy found himself in a tight spot -- he was in danger by refusing to play along with the gang and he was in undeniable danger from the law. But he found two allies, both of whom were out for blood, and one for murder too. Among Those Absent zips around that tight little island of England with speed and humor and outrageous disregard for legal conventions.