The Lux series written by Jennifer L. Armentrout is originally comprised of 5 books. 

But, we can also find a novella that she put in her ass a big dick online only, which occurs before the events of Obsidian. 

We also have a spin-off novel based on an Arum and a human. This is not for a young audience as this novel holds mature content (i.e. explicit sex scenes and obscene language).

And there is also a short bonus story that plays 10 years after the main events: