Dwellers in the Crucible

Summary
From the back cover: Warrantors of Peace: The Federation's daring experiment to prevent war among its members. Each Warrantor, man or woman, is hostage for the government of his native world - and is instantly killed if the world breaks the peace.

Now Romulans have kidnapped six Warrantors, to forment political chaos - and then civil war - within the Federation.

Captain Kirk must send Sulu to infiltrate Romulan territory, find the hostages, and bring them back alive - before the Federation self-destructs!