The Last Frontier by Alistair MacLean
Doctor Jennings, a noted scientist in possession of a precious secret, has gone over to the Soviet Union. It is Michael Reynolds' mission to get him back.
To penetrate behind the Iron Curtain and reach his quarry will be difficult enough, especially with the fearsome Hungarian Secret Police watching his every move.
Then, from inside that terrifying organisation, comes the offer of help - the question is: can it be trusted?