I read these novels a long time ago. It's definitely more than one book and if I remember correctly there's 3 novels. It's about a girl who is a median in some sorts and can transfer to another realm filled with magical creatures. She sometimes takes her friends too. She is also in love with a gypsy boy and in the end the boy gets stuck in a magic tree in that realm. Basically that's all I remember unfortunately I can't remember any of the characters names.

Sounds like the third book in Libba Bray's series The Sweet Far Thing (http://en.wikipedia.org/wiki/The_Sweet_Far_Thing)