There are three stages to the metadata download process:
1) An identify stage: this uses isbndb and google books to get the book isbn from title/author or get the title/author from the isbn. This also queries the cover download plugins has_cover method
2) Social metadata download: This downloads tags/rating/comments/series based on the isbn from step 1
3) cover download. picks the first cover returned by the cover download plugins based on the metadata discovered so far. All builting cover download plugins use isbn.