Things Hoped For (Andrew Clements)

In Things Not Seen, the prequel to Things Hoped For, a young adolescent boy (Bobby) suddenly, for no explainable reason, goes invisible. Eventually he returns to his normal self physically, but he is forever changed as a result of his time spent unseen.

Suddenly Bobby is 18 and now prefers being called Robert. His new friend Gwen has run into some massive trouble of her own. Her grandfather has suddenly vanished without a trace, and Gwen struggles with worrying about him and the pressure of her upcoming auditions for college music scholarships. She quickly learns that Robert is the best friend she can have when dealing with things not seen.

Another winner by Clements, but it's a bit more mature (not rated R or anything) than his previous works.