Okay so I LOVED this book and I was so excited for the release. A book that focused on Nico as an individual (rather than having his own side quest and doing stuff behind the scenes within the main plotline) was an addition the pjo series needed. I'm so happy uncle Rick is giving his most interesting side characters their own development and own series that focus on them as the main character rather than an accessory.
Nico was an obvious (yet very welcome) choice for this development in the pjo universe.
Nico and Will have been beloved characters for a good while within the series and I love how this book draws attention to both the harmonies and hardships of their relationship. It isn't afraid to show how they sometimes clash as individuals but both have an understanding that it doesn't make or break their relationship. If anything: their differences complete one another.
However 
It took me a while to get through simply because it was co-written. And while that wasn't a bad thing nor is it a deal breaker: I'm so used to and fond of Rick's writing style that it was sometimes jarring when Mark Oshiro was writing instead. I know, from friends that have read this book, that it isn't an issue for most people. But to me I found the two authors have different writing styles (especially when it comes to describing settings or emotions) so I sometimes found it hard to get into long reading sessions of this book as the difference broke up the chapters quite a lot to me.
Mark Oshiro was nevertheless an incredible choice of co-author for Rick to make as I did enjoy his individual writing style but I think (since I'm very comfortable with Ricks writing style and I find it easy to read) it simply took longer to read/ follow than a normal Rick Riordan book for this reason

Was this the best written book I've ever read? Not necessarily, no. But I couldn't put it down for the last half of the book. I loved how Nico and Will were able to work and grow throughout their journey, especially with Nico's past trauma and subsequent PTSD. Part of the core of the story is them learning how to open up to each other, how to work with both of their pasts and move forward as a couple. 

The care taken with the mental health rep overall in this book is wonderful. Will is learning that Nico doesn't need to be fixed, and Nico is trying to learn how to move forward with his darkness. They are so wonderful together. I am so happy they got their own book, and that Nico finally got his time to shine. I would definitely read another book with them at the center!

This is the book that was written specifically for me.
I can't even put into words how it feels to go from Nico being a favorite character, to him becoming more and more beloved and relatable for me, to him becoming a protagonist and going on a journey that struck me terribly deeply. Nico and Will's story is not about falling in love, it's about going from "in love" to loving each other fully and truly. And I've never, NEVER seen that written with such a powerful authenticity. Every emotional interaction, every little misstep, and every little progress, felt like a punch right in the gut. I'm mentally ill, I have a relationship, and this is EXACTLY what it's like. Nico being scared that letting Will help him will scare Will away or put too much strain on him. Will not knowing how to reach Nico and how to really help him when he's struggling. Them having to accept parts of each other that are so terribly hard to open up about. It's so real and raw and not at all what I would have expected when I first got into Percy Jackson all the way back.
And at the same time, it still IS a PJO book. It's quirky and there are silly incarnations of old legends and monsters, there's adventures and sudden troubles and a touch of mysterious force from gods that more and more are becoming a reflection of their concepts. I was having fun and processing emotional trauma at the same time. What more do you want. This is probably one of my all time favorites now.

This book definitely has moments where it works. The emotional heart is strong. The non-linear storytelling is goofy but it works in theory. And I enjoy Nico and Will and several of the side characters and some of the humor. But it's definitely kind of artless in its execution and there isn't as much charm and quirkiness and there usually is in a Rick Riordan novel. The plotting is much more competent and coherent than in the Heroes of Olympus series, but that's a pretty low bar. And a lot of the references and jokes were just too cringey for me. They might be less so for someone who's actually currently the target audience. I'm glad that the youth of today get gay cringe about Nico di Angelo, but ultimately that's what this book is.