Emerald has a special role in the Bookstore Demon association. In all her centuries of existence no demon has deigned to make her their partner. She has never lacked for company however, but love is an entirely different matter.
Many things have changed since the Slayer joined the company (see novella 1 in this series) and now more changes are going to occur.
It is a matter of some argument whether these changes are for the better for all involved....where there's smoke there's bound to be fire - or a hot demon.