This is a beautiful book -- not too treacherous. It opens with Jenny and her brother Tom returning home through a forest. Except that as he plays his flute, the forest comes to swallow him up. He screams for her to flee.
Seven years later, she comes back.
Needless to say, on top of the family distress caused by his disappearance, no one believed her. She tries to visit the woods again, to gain some closure. Which is when she finds that Tom is in there. Chasing after, she finds herself deep in the forest, and a boy perched in a tree suggests she ask the Foletti -- small, winged, beautiful as butterflies. When she tries that, they lead her off a good clip, and end up having her elfshot.
A young man finds her there before she passes out, and from his point of view, we learn he, Jack, is the guardian of the edge. It's his business to see her out again. Unfortunately, she had told him she was looking for her brother. And he's also oath-bound to help those on quests -- as Puck reminds him.
The tale then goes on to invoke a Woodsman and his Goodwife, who had had a daughter named Hannah, Jack's calling her Jenny Wren, Jenny's talking to a tree, a crown made of flowers, her clothing getting washed, the true reason for the existences of jack-in-the-boxes, Jenny's being caught in a net, a piper who can make storms arise and roses entangle a foe, and much much more. It's not a re-telling of Tam Lin, but you can see the echoes. Also of Norse Myth, Sleeping Beauty, and King Arthur in a beautiful and intricate tale.