Image Object
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    The image object is a child of the **book** object in the key ``files``.

* **cover_image** *<string>*::

    A dictionary with two keys: ``filename`` and ``url``.
    The URL is the path to the image on RedShelf.com.

* **thumbnail** *<string>*::

    A dictionary with two keys: ``filename`` and ``url``.
    The URL is the path to the image on RedShelf.com.