This is an extrapolation of events that happened at the 3rd season episode "All our yesterdays" where Spock met Zarabeth in the dead planet Sarpeidon. Since we saw a very satisfied-looking, almost kittenish Zarabeth at some point in the episode, fans started to infer that well, boy meets girl, it's bitterly cold, there's only one set of furs... you know! *wink wink*
The novel is well written and you do end up liking Zar.
BTW, this is the first book of the 'Yesterday Saga'.