This is the new title by Timothy Zahn, published by Titan Books.
The product description notes: “In Terminator: Salvation, the highly anticipated new installment of the Terminator movie franchise, set in post-apocalyptic 2018, Christian Bale stars as John Connor, the man fated to lead the human resistance against Skynet and its army of Terminators.
From The Ashes, written by Hugo Award-winning author Timothy Zahn, is the official prequel novel to the high-octane new movie, revealing the full story leading up to the movie’s events.
In post-Judgment Day LA, two lost kids named Kyle and Star keep watch for Terminators; a jaded Marine struggles to keep his rag-tag community together in the face of unrelenting danger; and John and Kate Connor assemble their Resistance team for a brutal assault on a deadly enemy.”
More info can be found here: http://www.titanbooks.com
The book finishes at 320 pages and is available as of March 17, 2009. Visit the website and Amazon for more details.