The Interlopers — Donald Hamilton

This time Mac has gone too far. For Helm to impersonate a Communist courier to whom he bears no resemblance is suicide, and that isn't all. Someone wants to get the layout of the Alaskan North-west Coastal Defence System, but they aren't the goodies or the baddies. So who the hell are they? And who is the mysterious Holz that Helm is meant to kill?