How do I tag books at Amazon?

Amazon has actually discontinued the use of tags; any that you've created in the past can still be accessed through your profile, but you can't add new tags to a book listing and they won't be displayed on the listing anymore. Amazon is constantly testing new features and frequently rolls these features out for a small segment of their audience before making it available to everybody.  Instead of tags, they recommend that you use  Wish Lists, Customer Reviews, and Recommendations instead.