What is Maniac Magee's real name?

Maniac Magee's real name is Jeffrey Lionel Magee.
Maniac Magee is introduced to readers as Maniac Magee, but the book quickly mentions that "Maniac" is not his real name. Jeffrey Lionel Magee is his real name, and the book spends the first seven chapters explaining how Jeffrey is eventually given his unique nickname. As Jeffrey runs into the town of Two Mills, he interacts with a bunch of different kids and groups of kids. Jeffrey's interactions defy belief. He intercepts a football and punts it back all while holding a book in one hand. He rescues Arnold Jones from Finsterwald's backyard. He even manages to hit home run after home run off of John McNab. Finally, Jeffrey manages to get a home run by bunting the "frogball" that McNab pitches.
Nobody in Two Mills can quite believe what they are witnessing. Their stories just don't seem to make sense, and they come to the conclusion that the kid must be some kind of "maniac."

Nobody knows who said it first, but somebody must have: "Kid's gotta be a maniac."
And somebody else must have said: "Yeah, reg'lar maniac."
And somebody else: "Yeah."
And that was it. Nobody (except Amanda Beale) had any other name for him, so pretty soon, when they wanted to talk about the new kid, that's what they called him: Maniac.
The legend had a name.