Book tagging algorithm is an algorithm based on machine learning and natural language processing technology, which can automatically analyze the contents of books and generate corresponding labels for them. These tags can help readers better understand the theme, content and characteristics of books, thus providing more accurate recommendation and search results. Through semantic analysis and keyword extraction of book text, combined with big data and user behavior analysis, the algorithm determines the most suitable tag. In this way, readers can quickly find books that suit their tastes according to their own interests and needs. Book tagging algorithm has been widely used in libraries, online bookstores and reading communities to provide readers with more convenient and personalized book recommendation services.