The purpose of the study was two-fold: to investigate the effects of a group-based book reading plus self-efficacy and outcome expectancy intervention on reading amount, perceived reading ability, reading behavior, and social cognitions and to examine influencing factors of book reading behaviors. Sixty-three sixth graders completed a ten-week book reading intervention in two experimental conditions: one with book-reading activities only and the other with a book-reading plus social cognition intervention. The control group (n=32) studied English in their regular English classes. Questionnaires were administered three times to assess the participants' social cognition, reading behavior, and perceptions of reading English books: right before and right after the intervention, and at eight weeks post intervention. The results showed positive effects of the intervention on reading amount, perceived reading ability, and reading behavior. Of the two experimental groups, the group that had received the book-reading plus social cognition intervention showed more positive results. At eight weeks post intervention, intervention effects on goal efficacy, reading behavior, and attitude toward reading were maintained. Social cognition was found to affect reading amount and behavior for the experimental groups. Research and practical implications are suggested.