When does Percy die?
Percy-Boyd Staunton (later shortened to Boy Staunton) does not die until the third book of the series World of Wonders. He is killed, or is suggested into killing himself, by Magnus Eisengrim. Magnus Eisengrim was once a little boy in the same town as Percy-Boyd and Dunny, and it was his mother, Mary Dempster, whose madness was caused by Percy-Boyd all those years ago.
When Dunny and Percy-Boyd were coming home from sledding, Percy threw a snowball (later revealed to have a rock inside it) at Dunny. Dunny ducked, and the snowball hit the pregnant Mary Dempster. She had her premature baby that night, and was never right in the head again. Eventually she sunk into complete madness, and was cared for most of her adult life by Dunny. Percy never took responsibility for this action, and it is not entirely clear that he even really understood his culpability.
Magnus (little Paul Dempster, then) ran away from home and joined the travelling circus, because of his mother's disgrace and madness and his father's controlling ways. There he was horribly treated and lead a life which he blamed on Percy (he didn't fully understand whose fault it was until the night he met Boy Staunton as an adult). Magnus says that he only suggested that Boy Staunton kill himself. Magnus was such a consummate magician and hypnotist that he may have been able to do such a thing, but it is also possible that Magnus somehow made Boy drive his car into the water. It is not entirely clear at the end of the novel what happened, but it is clear that Magnus is in some way responsible for Boy's death -- just not how much!