Is there going to be a sixth book of Percy Jackson Rick Riordan?
No. The series ends with the fifth book.
Yes, there will be a fifth Percy Jackson book. It comes out in Spring 2009. It will be the last in the Percy Jackson series, but Rick Riordan is going to write more stories about demigods, with different main characters. But it will still have some of the main charactors from the Percy Jackson series.
There is not a sixth Percy Jackson and the Olympians book. The Percy Jackson series has been concluded. However, there is another Camp Half-blood series that Rick Riordan is writing titled 'Heroes of Olympus'. This new series will have some of the old characters (including Percy Jackson) from Percy Jackson and the Olympians. The first book of the new series is titled 'The Lost Hero' which will be released October 12th, 2010.