I think probably my favourite part of this book is its atmosphere. For me, some books have a sort of mental smell, which I associate with the book in my mind. Heinlein's books often smell like my father's garage, all metal and grease. The atmosphere in Into the Forest is so alive it's breathtaking. You can smell the forest in the pages, the rich loam, the living smells of life and decay. It's sharply real, while still maintaining a sort of dreamy quality.
I liked Nell as a character. She's young and kind of silly, but she gets her shit together fairly quickly, which is nice. Her sister is a bit of a tit, though.
All in all a magnificent read, and has only two flaws: it's too short, and it doesn't have a sequel.