THE SUMMER OF THE PAYMASTER by Alfred Nielsen (W. W. Norton: $19.95; 379 pp.) This riveting book is easy enough on the surface to dismiss as another "coming of age" novel centered around a group of boys growing up in the late '60s on Staten Island. But the summer of '68 is a critical one as Andrew (the Chun) Hapanowicz--toiling as a pump jockey at Pete's Mobil station next to the world's largest garbage dump--eagerly, but apprehensively, awaits the return of his special best friend, Jimmy Dietz, from Vietnam. The mystic paymaster of the title, a roving Mobil Oil representative who prowls the company's filling stations and rewards pump jockeys for outstanding work, gives some purpose to the Chun's life at a time when his self-worth is under a severe strain. These are tough and complex kids, makings for a strong debut novel.