Diane Duane writes the most alien aliens in the Star Trek universe. She’s written the Horta, a race of glass spiders, and a second species of talking rock. She questions the relationships between these species and humanity, the Federation, and the fabric of space and time. She writes about them in incredible detail. By the time she’s done, you know how they think of themselves, what they think of you, and what they regard as tasty snacks.
In her 1984 novel, My Enemy, My Ally, Duane took on the Romulans. And although it’s really not unlike a lot of Duane’s other work on other alien races, it’s a stunning demonstration of what she can do.