Set in 1988 and 2013, the second novel by former Chicagoan Adam Levin (“The Instructions”) takes place in a world in which the internet doesn’t exist. Instead, everyone is obsessed with their robotic pet, or “Curio.” Belt Magnet, a maladjusted 38-year-old who lives at home with his widowed father, was one of the first to own a Curio and now struggles to interact with the outside world. “Bubblegum” finds him forced to confront it.