The answer to this question can be found in chapter 5. This is early in the book, and the purpose of this chapter and sequence is to continue building the legend that is Maniac Magee when he first comes to the town. Arnold Jones is a boy that got himself caught in a bad situation. He was found, alone, in front of Finsterwald's house, and the high school boys that saw this decided to toss Jones into that particular yard.
Arnold Jones was being hoisted in the air above Finsterwald's backyard fence. The hoisters were three or four high school kids. This was one of the things they did for fun. Arnold Jones had apparently forgotten one of the cardinal rules of survival in the West End: Never let yourself be near Finsterwald's and high school kids at the same time.
Jones is so terrified to be in that yard that he goes completely silent and becomes practically catatonic.
As for Arnold Jones, he clams up the instant he hits the ground. He's on his knees now, all hushed and puckered. His eyes goggle at the back door, at the door knob. He's paralyzed, a mouse in front of the yawning maw of a python.
Maniac then comes by and calmly opens Finsterwald's gate, walks in, picks up Jones, carries him to the front steps, and waits with Jones until Jones comes enough to his senses to run away.
The phantom Samaritan stuck the book between his teeth, crouched down, hoisted Arnold Jones's limp carcass over his shoulder, and hauled him out of there like a sack of flour.