Why did Maniac Magee keep Amanda's book in Maniac Magee? (Ch 6-12)

Maniac is fascinated with Amanda’s book and keeps it to reread.

Amanda keeps her whole library in a suitcase.  Maniac is very impressed when he sees all of the books.  He asks Amanda to borrow a book, only one, even the shortest one.  When he asks for her address and promises to return it, she finally relents and tosses him a book because she is late for school.

The book came flapping like a wounded...

Maniac is fascinated with Amanda’s book and keeps it to reread.

Amanda keeps her whole library in a suitcase.  Maniac is very impressed when he sees all of the books.  He asks Amanda to borrow a book, only one, even the shortest one.  When he asks for her address and promises to return it, she finally relents and tosses him a book because she is late for school.

The book came flapping like a wounded duck and fell at Jeffrey's feet. It was a story of the Children's Crusade. Jeffrey picked it up, and Amanda Beale was late to school for the only time in her life. (Ch. 3)

Jeffrey (Maniac) has to do everything with one hand now that he has the book.  This barely slows him down.  He shows up all of the other kids playing sports one-handed.  He is enamored with the book.  Even though he is able to read it quickly, he can’t bear to give it up.

He started reading Amanda Beale's book his second day in town and finished it that afternoon. Ordinarily, he would have returned it immediately, but he was so fascinated by the story of the Children's Crusade that he kept it and read it the next day. And the next. (Ch. 8)

Maniac has clearly missed the elements of normal life.  For him there has been no school, and no books.  He loves the story and rereads it passionately.  Maniac brings the book with him everywhere, and is able to keep it in perfect condition until Mars Bar attacks him and takes the book from him.

Maniac is horrified by Mars Bars’ defiling of the book.  A page rips out while the boys tussle over it, and he doesn’t want to return the book to Amanda in that condition. Maniac tells Amanda that it was Mars Bar’s fault.  She kicks Mars Bar for hurting her book.

Maniac is able to find a small amount of normalcy with Amanda.  Even though she is black and he is white, that means nothing to him.  He cares about Amanda, and the friendship is formed by her lending him a book.