The book was very much inspired by Foucault’s work, even if Stuart Elden takes issue with what Foucault says directly about territory. For an interview with Stuart Elden which provides an overview of the book see the Exploring Geopolitics site
I’ve just received my advance copy of The Birth of Territory, and additional copies have been sent by the warehouse. The Chicago page for the book has the book listed as published, and orders from them direct will be sent now (there are also electronic copies available). I guess online bookstores will take a few more days to receive copies.
[Update 9th September: the Kindle edition is now available. Amazon have the physical copy down for release on 13th September]
Foucault News 