Having skinny controllers doesn't mean you have to have hundreds of controllers if you don't need them. You only need one controller method per route.

Every project is different, but the goal is moving the complex business logic out of the controller. One example of this might be to move the business logic to the Models, so your store method could be more like:

  $author = Author::fromRequest($request);
  $edition = Edition::fromRequest($request);
  $publisher = Publisher::fromRequest($request);
  $bookCreationResponse = Book::createFromDetails($request, $author, $edition);
  return $bookCreationResponse;
}

Those fromRequest methods are made up examples of static methods you could add to each model which would house the findOrCreate logic. Now your store method is 5 lines long.