The 'Rihannsu' Series

[squiggy]

There is quite a few of them, spawned off by the wonderful Diane Duane novel "The Romulan Way", where thier' proper name was introduced.
The Rihannsu are actually very fascinating people, however, the most recent book, "The Empty Chair" was very thought provoking. Plus the re-use of older characters both Cannon, and some of Diane's own designs, including K's't'lk, a fascinating glass spider, or Arrhae, a human spy placed into the Romulan homeworld, makes these stories very interesting.
The best part about them, is that there is no Cannon to directly contradict any of it. I've checked.
Probably the most fascinating, however, is the Alliance with Ael, and the crew of the bloodwing, and the amazing way that Kirk's crew and Ael's crew worked as one, rather then two. Perhaps proving that one day, the walls between them will be shattered.