I thought it was really good addition to the pjo universe. I loved learning more about these two characters and their journey together in this book was sweet and enjoyable.
The only reason I deduct a star is for the level of difficulty/hardship for the characters was lacking later on as the physical situation got more dire, yet had a strong presence earlier in the story which seemed odd to me (trying to avoid spoilers). Personally I thought considering the danger, that sense of danger should still be present if not worse (still keeping within a childrens style book), just present the danger more gradually, however I also understand how the characters were adapting to the situation and having the benefit of each other's company. Therefore, plot-wise and for emotional-arc reasons, this likely done purposefully and not an oversite in editing.
Regardless, I still loved the book and reread it a few times for fun. I loved Nico from the PJO and HoO series, he was always my favourite character reading them since Titan's Curse, so a book with him as the character was awesome to read.