My Enemy, My Ally remains one of the best classic Star Trek novels ever written, even thirty years after its original publication date. I want to give this one to everyone I've ever known who's been disappointed by a Trek book and say, "Read this. It will change your mind about Trek."
I will admit right up front that I have a rather large bias toward Diane Duane, who has written a considerable portion of my favorite books (her YA fantasy series, Young Wizards, is also very much worth your time to check out -- but let's not digress). She has a distinctive, lyrical, descriptive prose style that makes each scene jump off the page, and a gift for choosing exactly the right words to evoke specific images for the reader. Specifically in terms of Star Trek: she writes aliens very well. The television shows tend to stick to humanoid races out of the necessity of using human actors, but since literature has no such restrictions, Duane's aliens are as strange, interesting and unusual as one could imagine. My favorite of her original alien races are the Hamalki, who are essentially glass spiders who communicate by singing.
Enemy/Ally in particular is the beginning of what would eventually become a pentalogy (or tetralogy, if you prefer, since the third and fourth books were intended to be a single volume but were split in two by the publisher). The novel centers around a high-ranking Romulan officer, Ael t'Rllaillieu, who is an old off-and-on enemy of Captain Kirk's -- and, if you're familiar with the show, the aunt of the female Romulan commander from "The Enterprise Incident," which further puts her at odds with Kirk and company. Ael has learned of experiments taking place on a remote space station, sponsored by the Romulan government, which involve forcibly taking genetic material from kidnapped Vulcan test subjects in order to attempt to create a method for Romulans to be able to use the Vulcan telepathic disciplines. While loyal to her people, Ael is also a highly honorable woman with a strong sense of morals and ethics, and the knowledge of what her government is becoming -- seeing the growing corruption in the Senate, and knowing to what use the mind disciplines would be put if the experiments are successful -- serves as the proverbial straw that breaks the camel's back. Unable to gain any help from her allies in the government, and more or less exiled to a tour of duty in the Neutral Zone where the Romulan government hopes she will get herself killed, Ael has no choice but to betray her people and turn to her old enemies for help.
I could write a million pages about Ael: she's mature, experienced, competent, able to match wits with Kirk and Spock, and strongly present in the story without upstaging or overshadowing the canon characters. Her relationship with the crew of her ship, Bloodwing, parallels in a rather lovely manner the familial relationship that the crew of the Enterprise have with one another. On the other end of the spectrum, she isn't immune to making mistakes, misjudging others, or failing to see things coming -- in a couple of cases, quite tragically so. In short, she's a well-rounded, dynamic character, and a strong female protagonist in a series (and, let's face it, genre) that sometimes ends up short on such characters. When I first read these books when I was young, I took to Ael immediately; she was one of my first real literary role models, and I'm very, very grateful to Diane Duane for bringing her to life.