The Schwa Was Here by Neal Shusterman (Farrar, Straus & Giroux, 2004) is a very, very good middle grade novel. I loved the main character's voice, his attitudes, how he talks about New Jersey I know I'll reread it, because I'll want to spend time with Antsy again.
"These are not books, lumps of lifeless paper, but minds alive on the shelves."
-Gilbert Highet, writer (1906-1978)
Here's a brief introduction to the story:Anthony (Antsy) has discovered his classmate Calvin Schwa has a magical ability to be invisible. The two band up to make money out of the dares other kids challenge them to--i.e. the Schwa can sneak into the teachers' break room and listen to what's going on without getting caught or even noticed. Their money making scheme works well until they get caught by Old Man Crawley.
To read about the author, go to: http://www.storyman.com/