You could use a custom column for marking books as favorite (e.g. a yes/no column) and save a search for that for quick access. Or just use a tag for that.
Additionally, you could use column coloring, column icons and/or cover grid emblems to easily see which books are your favorite ones.
I use the rating column for that (but I don't download ratings in metadata download, I rate only the books I've read). I have a saved search for books with ratings 3-5 and an icon and an emblem (heart - not very original ;P) for books with ratings 4-5.
If the rating column is used for external ratings, an additional custom rating column could be created.
And the Reading List plugin is a great idea, if it better suits your needs (I use it for my to-read lists and the list of books on my reader).
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