Introduction: This study examined whether user ratings, expert ratings and the purpose of the use of a book on a user-generated site influenced the credibility of the book. It also examined whether the effects of user ratings and expert ratings on credibility judgments of the book varied according to the purpose of information use. In addition, this study examined whether the co-existence of user ratings and expert ratings influenced people's credibility judgments of user ratings, and vice versa, on a user-generated content site. Method: Data were collected using both an experiment and a survey. The experiment took the form of a 2x2x2 factorial design, involving user ratings of a book (high or low ratings), expert ratings (high or low ratings) and purposes (academic or casual reading). A total of 233 college students participated in the study. Analysis: Analysis of variance (ANOVA) was used to test the research hypotheses and answer the research questions of the study. Correlation analyses were performed for the exploratory findings. Results: Expert ratings influenced the credibility of the book while user ratings did not. The effects of user ratings and expert ratings did not vary according to the purpose of information use regarding the credibility of the book. However, when expert ratings were high, the respondents perceived higher user ratings as more credible than lower ones. On the other hand, when expert ratings were low, the respondents perceived lower user ratings as more credible than higher ones. Conclusions: Expert opinions are important in the credibility judgments of the book and the credibility of user ratings on a user-generated site. The results provide practical implications for user-generated content designers and librarians.