Honestly, it wasn't as good as I'd have liked it to be. This was co-written and it SHOWS. The writing style is so different from Rick Riordan's. The plot is a mess, littered with typos, and isn't at all consistent with the Canon set in previous books.
As other comments have said, it feels like a fancier (derogatory). Will and Nico become caricatures of themselves, with the plot getting buried under their angst. While angst is not necessarily a bad thing, when the plot has so many plot holes it rivals the moon, there's a problem.
Spoilers:
1. Where'd Damasen go? What happened? Why did Bob reincarnate but not Damasen?
2. Why could Hades suddenly summon Bianca and Maria? While I understand Nico deserves some closure, breaking the rules set in previous books seems like it makes everything that happened in previous books no longer matter.
3. For some reason all the campers EXCEPT Will and Nico leave camp Half-blood. How convenient for the plot, but canonical it makes no sense whatsoever. Okay, it's nice Nico and Will get to be together, but that just implies ALL the campers just... feel safe outside of camp? Weird.
4. When was Nico close to Piper to the point that he calls her up and has a heart to heart with her? Honestly it would've been more convincing if he had called up Reyna. But PIPER? I understand the need for representation, but it feels so shoehorn in that it loses the rationality of plot.