Hawksbill Station - Robert Silverberg
The authorities have come up with an unconventional but effective way of controlling dissidents. Send them back a billion or so years into the past. A bit hard to escape from there, really.
When a new prisoner is sent back, the current top dog, an aging main with a recent serious injury has to try and hang onto his life, and work out what is up with the new guy.
3 out of 5