Dwellers in the crucible : star trek: the original series, book 25

The Federation is conducting a political experiment with the Warrantors of Peace, a group of hostages for their governments held safely on a protected area of Vulcan as long as their homeland remains at peace
DWELLERS IN THE CRUCIBLE

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 that world breaks the peace.

Now Romulans have kidnapped six Warrantors, to foment 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!