Educational reforms in Mainland China in the 21st Century reflect fundamental changes in principles and concepts in education. Such changes are also reflected in the ways textbooks are written. Modern theories of curriculum design hold that a textbook should not merely be an informative tool but should also serve as a "learning guide" that helps students to explore and acquire learning methods. Based on a case study of the "Higher Chinese Language for Secondary Schools" Textbooks in Singapore, this paper intends to draw out the basic principles of textbook writing in order for language textbooks to better fulfill the role as "learning guide". With a detailed explanation of the principles based on a theoretical framework, this paper aims to serve as a reference for textbook writers, teachers and students.