The purpose of this exploratory study is to understand book tagging by investigating LibraryThing (LT) members' purposes for tagging; exploring how tags are used; and comparing member assigned tags with terms in corresponding MARC records. The results show that LT members tag mostly for personal reasons, especially to manage their own collection, but they also tag to assist others to find a book. Once assigned, LT members use the tags to search and retrieve books, to gain information about books, and most importantly, to assist with personal collection management. Contrary to users of other popular tagging systems such as Flickr, LT members do not perceive social networking as an important factor when assigning and using the tags. The study also reveals that book tags by users vary greatly from MARC records. When tags overlapped with the terms used in MARC records, the majority of the matching occurred in the fields of 600s (Subject Access Fields) and 245 (Title Statement). The findings of the study contribute to the understanding of how books are represented in LT members' conceptual spaces, and have implications on how libraries could employ user created tags in OPACs.