The only negative aspect of the book is the failure to tie of a story line. Because of the floating questions there needs to be an epilogue. If he does create another series then there would be no need for an epilogue but until that day it simply needs to resolve one more story line.

The Blood of Olympus by Rick Riordan was a decent conclusion to the Heroes of Olympus series.  It tied up most storylines and was well written using a good set of vocabulary and ancient terms. I can see Rick Riordan making another series off of this one similar to how he created The Heroes of Olympus off of the Percy Jackson and the Olympians series.  In addition,the prophecy was well interpreted to have a happy ending.  The rise of Gaea just did not build up to the hype shown throughout the previous books.  It is a darker book similar to The House of Hades.  It puts a spin on the story of Orion and Lychaeon lore and they return from the first book The Lost Hero.  Every important character throughout the series is either mentioned or actually appears throughout this book.  It is truly an epic final book for an epic series.