The Romulan Way is a paperback novel based on the original series, published in 1987.
Introduction from the book:
They are a race of warriors, a noble people to whom honor is all. They are cousin to the Vulcan, ally to the Klingon, and Starfleet’s most feared and cunning adversary. They are the Romulans - and for eight years, Federation Agent Terise LoBrutto has hidden in their midst.
Now the presence of a captured Starfleet officer forces her to make a fateful choice - between exposure and escape; between maintaining her cover - and saving the life of Dr. Leonard McCoy.
Here, in a startlingly different adventure, is the truth behind one of the most fascinating alien races ever created in Star Trek:
The Romulans.
via Memory Beta