For a basic introduction, it is difficult to beat Shih Tzu for Dummies by Eve Adamson. You might also consider checking out The Shih Tzu Handbook by Sharon Vanderlip or the Terra Nova book The Shih Tzu by Deborah Wood.
Then you need to get a book that deals with the history and tempermant of that particular breed. Or you can do a net search and get information about it.