Angel of Fire - Wendy Milton

Zach Brinkley is a prickly, fiercely independent boy with a chip on his shoulder. When this story opens he's in the hospital, having been hit by a truck in a pedestrian crossing near his home. His heart has stopped and he's clinically dead.
Doctors resuscitate Zach but he winds up in a coma, unable to communicate with anyone except some rather bizarre, otherworldly visitors. He meets Astra, for example, a yellowish "winged thing" who irritates him in the extreme. He also meets a set of scary twins who reveal to him that his being hit by the truck was no accident!
Someone is trying to kill him? Who could want him dead? And why? This story follows Zach's determination to stay alive with the help of Astra, the twins and his only worldly contact, the insufferable, know-it-all Sophie Ferguson. Will he emerge from his coma in time to thwart his would-be killer?