It’s actually a series of four books, but they all happen at the same time. I read them about fifteen years ago, so they’re at least that old, but I don’t think a whole lot older.
The premise is that there are four planets around one sun, and each planet has some sort of virus or something that mutates anyone who visits the planet. This mutation, which is different on each planet, kills the person subjected to it if they try and leave. The planets are therefore used as a giant penal colony, since the criminals have no hope of escape.
The plot revolves around some sort pf conspiracy happening on the planets. Since a spy can’t go to the planets and return, the authorities clone one of their agents four times, and send one clone to each planet, and then somehow extract his experiences back into the original agent.