you can get so many mixed emotions
from just finishing a good book series.
maybe happiness because it ended how you wanted,
maybe sadness because someone you loved died,
maybe relief because the character has finally evaded all the evil they were faced with.
but if the book series is really good,
depression will be a major emotion
because maybe everything ended happily
but you can never again experience the feeling of reading that series for the first time again
and that is heartbreaking