"Winter's Passage" is a novella that bridges the gap between The Iron King and The Iron Daughter. It picks up with Ash guiding Meghan back to the Nevernever to find Puck, prior to fulfilling her contract to return to the Winter Court. Grimalkin, the faery cat ad one of my favorite characters from "The Iron King," makes an appearance to escort the pair.
Meghan Chase is chased some more, this time by The Hunter. Meanwhile the romance between Meghan and Ash continues to develop, even as he warns her that he poses a deadly threat to her.
This is a quick read and not a necessity to follow the series. A good portion of the beginning of the novella recaps "The Iron King," but it does provide Julie Kagawa's usual combination of excitement and tension.