It's called "Cold Case" and it's the story of Molly Carpenter's first mission as the Winter Lady. It's a good enough story, it's just fairly unimportant as far as the series goes. There really isn't any new information, except a climactic reveal about the nature of the Winter Lady, which sort of makes sense in hindsight, but not in the slightest if you read any book featuring Maeve.

Actually, thinking about it, it might explain a lot about her, psychologically speaking, and add a new dimension of malicious behavior to her messed-up personality, as well as provide a motivation for her rebellion against Mab.

Anyway, this new detail seems to mainly be there for the effect it will have on the Harry/Molly relationship that is teased from time to time. I can't decide if it will be an obstacle, or if it is the sort of practical obstacle that will propel the relationship in a narrative sense, since it gives conflict to that relationship and can make the characters involved suffer some more, thus allowing the author to go ahead and pull the trigger on it.