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From the Help Center:
The Bookshelf Header allows you to add a custom, publicly-viewable message at the top of your PBS Bookshelf. You can introduce yourself - give an overview of the types of books you have - comment about a specific book you have Posted - invite members to join your Game in the Games Forum - or just say Hello!
How to add a Header to your Bookshelf:
To see your own header on your Bookshelf: