This is second series in the Percy Jackson books. They are called "The Heroes of Olympus". There will be five of these books as well. So far, only two are out :( The first book, "The Lost Hero," is about Jason, Leo, and Piper. Three new heroes we will learn about. Percy is mentioned, but only the fact that he is missing. He has been missing as long as Jason has been present and Jason can't remember anything before that period of time. They have a great mission to save Piper's Father, who has been kidnapped and another very important person, who has also been kidnapped.
"The Son of Neptune" is back to Percy who finds himself also in a different place, other than camp half-blood, with a whole new group of friends. He, Hazel, and Frank will also be placed on a new quest to kill the impossible, release a monster, to save camp all in a short period of time. Hopefully all of this will restore Percy's memory and old live.
These were both wonderful books! Loved every minute of them. I could hardly put them down. Now, I have to wait until this fall when "The Mark of Athena" will be released.
I have not read this one yet, but there is also "The Demigod Diaries" available. It is a book of several short stories of the various demigods, such as Luke Castellan, Percy, and Leo Valdez. Sounds like it could be fun too!
Book Four is Scheduled to be released Fall of 2013! (No name announced)
Book Five is Scheduled to be released Fall of 2014! (No name announced)