Pre-order: Amazon.com, Amazon.co.uk, Amazon.ca, Amazon.de, Amazon.fr, Amazon.es, Amazon,it, Amazon.co.jp, Forbidden Planet.
You can also read an excerpt, chapter one from the book, here.
The Simon and Schuster online catalogue also has a new entry; under the Gallery Books imprint there is now a listing for the first Vanguard book, Harbinger. Gallery Books normally do the larger format Trek books (plus the new ebook releases, but this is listed as paperback), so one assumes this will be a trade paperback reprint. It's listed with an August release.