The first book that Grayson read cover to cover was The Little Engine That Could.
Accordingly, Maniac Magee didn't help Grayson at all. We are told that the latter made an almost superhuman effort to complete the reading by himself. After Grayson was finished, Maniac let out a loud "Amen."
When the old man asked Maniac why he reacted that way, Maniac answered that it was perfectly natural to use the exclamation. After all, Grayson had just read the book cover to cover, a huge accomplishment for someone who had only recently learned how to read.
Later in the story, we learn that Maniac gives Grayson a book for a Christmas gift. The title of the book is The Man Who Struck Out Willie Mays. Maniac wrote the book himself in his own handwriting, and he drew stick figures to accompany the text.