This is probably the first book in the Percy Jackson universe to really feel like young adult rather than middle grade. It's heavily relationship focused with much lower stakes and less action than other books. I would actually describe it as a fantasy romance, albeit Will and Nico are already together at the start of the story. 

Their relationship is perfect. I loved it in previous books and having an entire book dedicated to them is an absolute joy. There are also going to be so many kids for whom having this book will mean so very much. 

The plot is shaky in places. This is the second book set in Tartarus, and it doesn't quite do it justice the way that The House of Hades does. The first half in particular didn't grip me as strongly as Riordan books normally do - I'm unsure whether that's to do with the writing (perhaps Oshiro's writing style doesn't work quite as well for me personally as Riordan's?) or whether it's because I started the book listening to the audio and only picked up the physical book later. Previously I have always read Riordan books physically on my first read. 

I absolutely adored the ending of the book. The last few chapters felt perfect to me (and had something totally unexpected which I loved). 

Plot-wise this is one of the weaker books in the series, but character wise it's among the very best. If you also love Nico and Will then run don't walk to it.