My favorite series is TOS and Romulans are my favorite aliens. I like relationship novels and am indifferent to sequel vs. standalone. I do prefer slightly heavier reading.
Well, everyone else has already chimed in with excellent recommendations, but it was my clarification request you've answered so i'll just add:
if you want an early Pocket novel with Romulans, try MS Murdock's "Web of the Romulans" (1983). I read it when it first came out and it's interesting in that Murdock spends as much time with the Romulans as she does with the Enterprise. At the time, that was quite unique for ST novels.