I think the book on its own could have been great but I spend the last week to read all previous 5 books and immediately read this one afterwards. It felt like a lot of thinks we're missing.
I found all of this generation slang really annoying. I guess the author was trying to write something that his target audience likes. I am 19 and I know I am not his target audience but I throughoutly enjoyed reading the other books. Always complaining about the older generations and that they would just leave and pass us all the problems that they created was a bit annoying. Percy has always been funny but it's just way to much and way to bad jokes. Percy was clever funny but not annoying predictable funny. Also I do not get why Percy suddenly hates the gods that much and tells so many bad thinks about them. I mean he never liked them but there was always some kind of respect or fear regarding other gods and especially his father. Who for some reason suddenly became really nice and even called his school??? Also Ganymed is the most god unlike and whiny being I ever encountered. Why is Percy suddenly so confused about Annabeths invisibility cap. As if he has never worn it. And the itching has never been mentioned previously. Why does Grover need dating tips and why is there so much hugging and stuff. Why is Grover so afraid of them moving to a different place. Why is Percy constantly so nice and worried about Grover's feelings and how he feels bad endangering him and that he needs to be protected constantly from even small dangers. The characters do not feel the same anymore. They feel misfigured and what they do appears wrong and out of character.
Others complained that the story that he has to collect those 3 recommendations is very unrealistic. I am personally fine with it but I think it is somewhat restricted. Traveling around the globe and visiting special places has always been part of Percy Jackson for me. Now everything needs to be close by. What about shadow traveling? One thing missing were also more stories from the greek mythology. I definitely preferred the second half of the book with focus on Gary. Some interesting old story newly interpreted. Funny names to guess what could be the story and a very clever solution to beat a problem. The first part boring talking to some gods and always a break where they continue with their day to day live was not so interesting.
What I really liked was the synergy between Annabeth and Percy. I was afraid that it would feel wrong that they are together now but it's quite the opposite. They behave like I would have imagined and are still a great team. It shows how much they have been through. I also like that Athene now somewhat supports Percy and the situation between him and his new family.
For the next book I would wish the characters to be more like they were before, longer and more dangerous and important missions. Not so annoying gods and characters, less children slang and to keep the synergy between Percy and annabeth.
The other books kept me reading without a break for food until 2am. But with this book I found myself to do more breaks and check my text messages regularly. To be fair it's hard to hold up to the legacy of the old Percy Jackson books and the book on its own is quite ok. Just not nearly as good as the originals. But I get it is really hard to write a sequel that is satisfying.