It's actually out right now. But on his web site (http://www.rangersapprentice.com/) he says that it will be out sometime in the spring of 09.
If your talking about the series with withches and spooks then The seventh book of The Last Apprentice series, A Coven of Witches, will be released April 27th of 2010. if not then the other book will be released sometime around may/june.
Warriors: The Fourth Apprentice will most likely come out some time in 2010. You can read up to chapter six on the Harper Collins website (see related links).
Your question doesn't make sense. There is a series called power of three but nothing called power of six. The next series is omen of the star starting with The Fourth Apprentice.
April 2011
Cloudkit becomes an apprentice at six moons old, as all Clan kits do when they reach the appropriate age and undergo their apprentice ceremony.
Out October 2011 x
Cherrykit and Molekit become apprentices at six moons of age, following the traditional Warrior Cats apprenticeship ceremony where they receive their apprentice names.
Sisters Grimm book six has already come out and is available in hard cover at most book stores. May 01, 2008
warriors book six is already out. But if you mean Warriors Power of the three book 6, It is called sunrise and comes out 08/04/2009
It already has been, or at least in America.
six
Power Rangers Megaforce - 2013 The Power of Six 2-9 was released on: USA: 2014