As an important means for preserving knowledge, various literatures have become precious resources for the mankind, which have greatly contributed to the social progress of human race.
Professional literatures have been regarded as "intangible assets" of the whole world because they are, on one hand, the summary, generalization, and development of the achievement obtained on the basis of previous experiences or studies, and on the other hand, they have been accumulated and handed down from generation to generation. In this sense, therefore, all kinds of literatures are records of previous research findings and academic achievements, and crystallization of human civilization.