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In Short
If you're looking for a standard YA paranormal romance, you'll probably be happy with Everneath. The book takes about 150 pages to get going, but it stays faithful to the usual genre tropes, such as having a love triangle, a persistently loyal love interest, and a mercurial immortal who mucks up normal life. If you're not looking for any of these things, you might not enjoy this book.

How I Explained This Book to My Mother
It's a reimagining of the Persephone and Hades story. This girl Nikki goes down into the Underworld (called the Everneath), and then she manages to escape back to the surface world… except this immortal guy Cole follows her and he's all "BE MY UNDERWORLD QUEEN," but Nikki says no because she's obsessed with her old (non-immortal) boyfriend Jack. Nikki then spends six months (sort of) figuring out how not to get sucked back into the Everneath. The moral of the story is that heroes do exist.

Writing Quality
Nikki narrates the book in first person. At her most melodramatic, she sounds like this (names removed to avoid spoilers, hopefully):

Separating from him now would be worse than anything I'd felt before. Separating from him now would make me bleed, and I would never stop.

I couldn't speak as to what occupied _____'s mind on that drive, but I knew what I was hoping for. That ____ would be able to recover. That he would heal. That those who loved him would soon repair the broken sheathing around his raw soul . . .

On the other hand, there are surprising and authentic moments to savor, such as these:

"And when something surprises you and you don't know what to say, you get a tiny wrinkle in between your eyebrows." I reached up to touch the divot, then hesitated and lowered my hands. "It showed on the day the coach told you you'd made first-string quarterback. And it's showing now."

_____ was looking at me with a strange expression on his face.
"Whoa. You are the happiest sad person I've ever met."
"Huh?"
"Or the saddest happy person." . . .
I tried a smile, so he couldn't see how close he really was to the truth. "I'm not sad."
"And she's not afraid to lie."

Do the Plot Lurch
This story is 370 pages and covers approximately six months of time in the surface world, including flashbacks here and there to a time before Nikki went to the Everneath. The book takes its time winding through the first four months, and then it tries to resolve all the reader's questions in the last 150ish pages. In the final weeks before she faces return to the Everneath, Nikki Googles a lot of weird things (i.e., "how to escape the Tunnels"), finally elicits answers from some people in town, and goes on a tangential quest to find the significance of a silver bracelet. The book spends a long time building up to answers—so long, I think, that the answers don't quite pay off in the end.

Two Star Reservations
Everneath gets a 2/5 from me because while the premise is original, the book avoids grappling with real emotional material that would have made it better. Nikki never deals with the true loss she's suffered in the past, nor does she make significant attempts to rebuild family relationships or friendships that she claims to value. I think she might have two meaningful (or almost meaningful) conversations with her dad, one coffee chat with her best friend Jules, and one afternoon with her 10-year-old brother. I get that this is a romance story, so it's fair for Nikki to be obsessed with Jack. But I wanted so much more for her—I wanted her to sit down and do at least some hard work of facing grief and trauma, of piecing together parts of her life that were as important as her relationship with Jack. Maybe I'm expecting too much of a genre book, but I'd have been happy if there were even 10 more pages of Nikki's other relationships. Is that too much to ask…?

Recommendations
Fans of the Vampire Diaries and the Twilight Saga will probably enjoy Everneath, even though there are no vampires. The books have heroines with similar voices, attitudes, and backgrounds, and the plots involve temptation by dark and dangerous immortals.

You can read the extended review and other book musings at The Matchbook.