Robert Comsol wrote:
I can't help but wonder why then it has not been recorded (see Spock's statement in "Balance of Terror"). Alternately it may just be a Romulan deception where in fact these want to avenge that their ancestors had been marooned on Romulus
First there can't be any "non-fiction" books on the Romulans, because they're a fictional race of peoples.
Spock not knowing anything about the Romulans right off hand could be because the Vulcans have been interstellar travelers for thousands of years. The Roumulan left Vulcan, but so did hundreds or thousands of other groups over the centuries. The Romulans departure might not have been considered anything special at the time and no important historical note was made of it.