Zelda was not merely being kind when she described her death as simply "moving to another branch of the Library", since she would be under the same unbreakable contract as Penny-40 whose term of employment extended for a billion years.
The expected reveal that Sir Effingham was really the Dark King in disguise (since both characters are played by the same actor) never happened.
Announced on March 3, 2020 that this will be the series finale.