Update: Christie Golden has written on her
official blog that
Twilight is set to come out April of 2009.
Book three of the
Dark Templar Saga by
Christie Golden is scheduled to be released May 1, 2010 according to most book retailers. The date has been pushed up from September 30, 2008.

Recently,
PopShopOnline updated their site with the back-of-the-book description of
Dark Templar Saga: Twilight:
After the seeming defeat of the dark archon on the Protoss Homeworld, archeologist Jake Ramsey is catapulted across interstellar space. But Jake does not have long to live - his enemies are regrouping, and Zamara's essence must be separated from Jake's mind before time runs out! His only hope rests with the powerful and legendary Zeratul, but as Jake is about to learn, even a dark templar can have a crisis of faith!
Sources:Wordpress - Christie GoldenStarCraft: Dark Templar: TwilightPopShopOnline - StarCraft Dark Templar Saga Twilight