Setting params for ABook can be useful to customize views in menus or for a single book (this one is present only for ABook version's for Joomla 1.6).
You can set general options in Components -> Alexandria Book Library -> Preferences.
Well, suppose that you published a menu item to show the whole library, but you have also published a menu item that shows "SciFi" genre and a menu item that shows all books for "Ken Follett".
You may have configured general options to show the links to download books. But suppose you don't want to show links for "SciFi" menu item. You can do that by opening the "SciFi" menu item and then, in Component Parameters disable External Link.
In Joomla 1.6, ABook give you the choice to change all settings also for a single book.
Then, you may go in Components -> Alexandria Book Library -> Books, open a book and change settings for this single book.
Nice, don't you think?