Along with the wide popularization of mobile terminal devices such as smart phones, mobile libraries, which provide mobile book information services as the core business, have become an important part of people’s daily life. However, with the rapid development of emerging network technologies such as cloud computing, the server side of a mobile library has become more and more untrusted, and the problem of reader preference privacy associated with mobile book information services has become increasingly prominent. To this end, this paper proposes to construct a group of false requests of mobile book information service at a trusted client of a mobile library, which are then submitted to the untrusted server side together with a true request, so as to confuse the reader book preference privacy. Firstly, we define a theoretical model for the protection of reader preference privacy. It measures the confusion effect of false requests on a true request by introducing the concepts of book information entropy, location information entropy, and book location joint entropy. It measures the concealment effect of false requests on readers’ mobile book preferences by introducing the concepts of book category distance and location region distance. Secondly, we present a specific implementation algorithm of the theoretical model to generate a group of ideal false requests that meet the privacy constraints for readers’ true requests. Finally, theoretical analysis and experimental evaluation demonstrate the effectiveness of the proposed method, i.e., it can effectively improve the security of readers’ mobile book preference privacy on the untrusted server side, without compromising the availability of a mobile book information service.