Side Character Becomes the Main Character

 The Sun and the Star by Rick Riordan 

Nico is the main character in this book, which frankly is overdue. It had all of the humor you come to expect from Rick Riordan, as well as the mythology, and adventure. Nico and Will are officially together and the have to go on a quest to save Bob the Titan. They travel through the underworld and have struggles. We meet Hiss-Majesty who is the best side character in a while and I will do anything for them. Will meets Persephone and gets some insight into Nico. The journey to Tartarus is fraught with peril, but they have help and are able to free Bob. He refers to them as his Sun and Stars and then never really explains that, so I didn't like that. Also, Nico is able to defeat Nyx through the power of growth. He also gets what he calls his Coco Puffs, which are physical manifestations of his fears, but he learns that just because he is darkness does not mean he is only darkness. He and Will grow together during this quest. Bob leaves them at Camp and journeys west because he can, because now he has a choice. Also, Mr. D was actually kind and helpful during this book, which was a huge change for him, but maybe he is growing too.  I love Bob and am glad we got to see him again. It was good that Nico finally got to take center stage and that we saw him come into his own. Another good addition to the Percy Jackson universe.