The idioms used to describe being addicted to online novels could be "addicted to the Internet","addicted to the Internet","fascinated","addicted", etc. These idioms all refer to a situation where a person is too obsessed with something or activity and loses control. For example, people who are addicted to video games can be described as "Internet fans","obsessed" people who are addicted to gambling can be described as "addicted" people who are addicted to online novels can be described as "addicted to the Internet".