I really wanted to love it, but some things fell a little flat for me.
To start, I really did love the prose and the vibes. It was beautifully written, and the descriptions were poetic. It felt like the setting itself was another character.
It was pieces of the plot that I had more trouble with. Not even plot holes, just like "oh, that's it?" moments.
Reid really did a great job setting up for a fantastic story. I think the initial plot line of 

the widow being the true author

was predictable just from the theme of

men exploiting women

that was established very early on. I genuinely don't think a predictable plot is a bad one though, and I had thought that Reid had created enough going on alongside this plot-line to really create a captivating story.
But then, nothing else really happened. There were some super interesting side plot setups:

Effy having some other worldly beauty to the point that even a random boy at the college was annoyed with her until he saw her face then promptly asked her out. Sirens and selkies were even mentioned throughout the story alongside with Effy being a changeling child, but nothing ever came of it.

The war was mentioned quite a bit, but always in passing, like it didn't actually affect anyone we knew. It seemed like the only purpose was for the Sleeper Museum to have a function, and/or for Effy to make vaguely racist comments to Preston when she was mad, but it felt very underdeveloped.

I did like that for awhile we're really not sure if Effy is crazy like everyone thinks or if the Fairy King is real, because even she isn't sure.
Some questions I do still have that people might be able to answer for me:
1.

Why didn't the widow talk to them at all? Why/How did she escape the hall without making a sound if she wasn't a ghost?

2.

The random sex scene felt out of place. It felt less intimate and more like the precursor to a 3rd act breakup

3.

Why was the estate guy so cagey? He was helping Preston one minute, but suspicious of them the next.