I finished reading The Son of Neptune, which is the latest Percy Jackson book. It's a good book. Percy Jackson fans should check it out.
The story of the book is that Percy makes it to the Roman Camp for demigods. He and two other characters are given a quest to go north to Alaska, where a giant is keeping Death captive. Our heroes go there, kill the giant, and return to New Rome in time to stop an army being led by a different giant.
Percy was my favorite part of the book, hands down. As for the two new main characters, I liked Hazel. Her backstory was kind of similar to Piper's in the previous book, in that both girls knew information about the giant ahead of time, but they wanted to keep it a secret. I think it worked better with Hazel than with Piper. The other new character was Frank. I wasn't sure about him, because he's sort of a mixed bag. That's on purpose, though; part of Frank's story is how he works through his confusion about himself and his family. My other Percy Jackson friend loved Frank, though.
I don't think I'd go so far as to say that it is immensely better than the previous book, which didn't have Percy in it, but I wouldn't be surprised if this one outsold the last book.