Hm, I feel like there are a few books where I would have preferred to be reading about the secondary characters/pairings, but I don't know that I'd call it a 'shadow' book, which seems to imply a genre or larger shift. (e.g. Crazy Rich Asians, the main pairing was boring to me, but some of the secondary characters were really interesting. But it wouldn't be that different a book, just on a different set of characters!)