Do Not Open by Kiersten Modglin

On the day author Mari Morgan receives the email, she has little left to live for.

The message—simple yet utterly life-changing—offers a glimmer of hope to her dismal existence. It’s the opportunity of a lifetime, and if she takes it, it could change everything.
Mari’s immediate sense of unease is eclipsed only by her curiosity—elicited from the supposed fan on the other end of the email. Unsatisfied despite doing her due diligence, and with no way to discern the legitimacy of the sender, Mari takes a leap of faith and replies.
What harm could come from one innocent message?
Soon, as she comes face to face with her ‘number one fan,’ it’s all too clear things are not what they first appeared. Now trapped by a villain far more sinister than she could’ve crafted herself, the fate awaiting her is inspired by the darkest depths of the monsters in her own novels.
In order to escape and survive, Mari must use the strength of the heroines she’s spent years writing about. But this villain has spent years inside her worlds, too.

How will she defeat an evil who has learned from the best?