The Secret Tree, by Natalie Standiford

Minty and her friend Paz love to roller skate.  They dream of becoming roller derby stars together.  And while their older sisters warn them that starting sixth grade may bring changes that will challenge their friendship, they are sure that they will stay best friends.  However, as the summer progresses it seems like those warning may come true even before school starts!  Suddenly, everyone seems to have secrets!

But the real adventure occurs when Misty finds a strange boy spying on her and her friends.  Tracking him when he runs away, she finds that he is living alone in an abandoned house next door to the crazy "Witch Lady" who strikes terror in the local children.  But it is his pastime - collecting other people's secrets - and how he does it (pulling them out of the trunk of a tree in the woods) that interests Minty.

If that synopsis sounds a bit odd, it's because the story is a bit hard to summarize.  It's an odd mash-up of realistic middle school drama and junior supernatural lore.  It's a story where your best friend's sister can be casting voodoo spells on your friend, but the biggest problem is that that same friend has failed to invite you to a pool party!  As crazy as this all sounds, it actually works.  There's just enough fantasy to seem magical and enough realism to be grounded.