One Thing Stolen by Beth Kephart

Nadia is losing it.  She's started stealing things - some cheap, some expensive - and she can't stop.  At night, Nadia makes birds' nests out of things she's found and stolen.  She can't stop making the nests, either.

Then Nadia begins seeing a boy running through the streets of the Italian city she and her family moved to a few months ago.  He leaves her flowers, Nadia's only proof of a boy nobody else seems to see.  Nadia keeps slipping in and out of memory, particularly ones of her friend Maggie back home in the US, finding it increasingly hard to stay in the present.

But worst of all, Nadia is losing the ability to speak.  Once-easy words fail to come out, or stop halfway through.  As her mind deteriorates, so do her relationships.  Every day, Nadia loses a bit of herself - and a bit of her ability to tell someone what's going on.

The race is on - will someone get her help, or will Nadia disappear completely first?