I wouldn't say that just including Spock makes something a TOS novel. After all, adding Worf to DS9 didn't make it TNG. And "Unification" was a TNG episode, not a TOS episode. TOS is the whole situation and cast, not just one member of it. Not to mention that Spock's ambassadorial role and involvement with Romulus are plot threads arising from TNG, so if anything, RBoE owes more to TNG than to TOS.
So I wouldn't call it a DS9/TOS novel; I'd call it a DS9/Ambassador Spock novel.