Are any new Rick Riordan books coming out?

Yes. New chronicle, Kane Chronicles - About Egypt Pharaohs and tombs coming to life and Heroes of Olympus chronicles. However, if you haven't read the Percy Jackson Series, I suggest you to read it. It is a very good book indeed! Also, there is another guide and a few short stories of Percy Jackson in 'The Demi God Files'. I hope this was the correct answer to your following question! :)