There's something of an adversarial edge to a lot of SetsuKarin depictions, and I'm not entirely sure where it's coming from. I'm thinking specifically of Setsuna's coldness here and in the other "SetsuKari Book", and in "SetsuKari Precious", where Karin's teasing has more of an edge to it, and Setsuna seems to be retreating to, well, more of a Nana mode. Maybe it's coming out of the start of season 1 of the anime?

That's a long way around to say I enjoyed this, but enjoyed Setsuna being flustered more than her being cold.