According to various findings from our research and related studies, library resources in general are not fully utilized by users. Since understanding users' interests is an essential step to promote uses of library resources, it is useful to analyze users' interests from some unobtrusive data like transaction logs. Therefore, in this paper, we used circulation history from Shinh Hsin University Library as a basis, which included a 2-year log containing over 170 thousands circulation records, to analyze users' interests. Association and clustering methods are used to discover relationships among books and books, books and users, and users and users. The experimental results show there exist valuable relationships among circulation data for various applications, such as being a basis of recommending new books for users by referencing clusters of classification numbers. Meanwhile limitations of the circulation data, automation system, data mining techniques, and privacy issues are also discussed.