Library Science is now popularly changed to be "Library & Information Science".Library science is an art as well as science.It is an interdisciplinary course/subject..In most of the universities it is considered as Arts subject.Library Science is also a subject at degree level in few Universities (Ex:Kuvempu University ,Shimoga).
While it may be an "art" for being a good librarian, library science is a "science" in the sense that it studies subjects like Information technology,Computer science, e-resources, Knowledge management procedures and other technical aspects to manage information/knowledge for easy retrieval and usage by the users of the library. That is what makes it a science.
According to my point of view it is neither purely art or science but a combination of the two.