Things Not Seen (Things #1) by Andrew Clements

Bobby Phillips is an average fifteen-year-old boy. Until the morning he wakes up and can't see himself in the mirror. Not blind, not dreaming. Bobby is just plain invisible...

There doesn't seem to be any rhyme or reason to Bobby's new condition; even his dad the physicist can't figure it out. For Bobby that means no school, no friends, no life. He's a missing person. Then he meets Alicia. She's blind, and Bobby can't resist talking to her, trusting her. But people are starting to wonder where Bobby is. Bobby knows that his invisibility could have dangerous consequences for his family and that time is running out. He has to find out how to be seen again before it's too late.