2005 (summer) - Lightning Thief
2006 (summer) -Sea of Monsters
2007 (winter) - Titan's Curse
2008 (summer) - Battle of Labyrinth
2009 (summer) - Battle of Olympus
Sometime in winter of 2009 (probably in start of December), Percy disappears.
2009 (December) - Lost Hero
2010 (June) - Son of Neptune
2010 (June-July) - Mark of Athena
2010 (July) - House of Hades
2010 (July-August) - Blood of Olympus
From January to June 2011, Trials of Apollo. But I wouldn't write on it. I didn't like this series very much.