Calibre is a full-fledged e-book library management tool with solid functionalities and support for numerous e-book formats.
Calibre, in Ubuntu, can easily be installed via Ubuntu Software Center, providing a clean interface with plenty of options and configuration options, nevertheless it lacks, by default, appmenu support, thus hovering the mouse pointer over the Unity panel triggers an empty panel (no menu is displayed).
Although the menu can be accessed via the big toolbar icons, there are users used with Ubuntu's appmenu approach, furthermore enabling the appmenu will automatically add HUD compatibility (hence, Calibre can be maneuvered via the HUD).
The solution is as simple as activating the built-in disabled Calibre appmenu: navigate to
Calibre-->Preferences-->Toolbar-->select The menubar-->add actions-->Apply (to add an action, select it and press the forward icon in order to populate the Current actions area)