The U.S. paperback edition of Unknown Quantity was published in May 2007 by Plume, an imprint of Penguin Books, New York.
The text included many changes from the original U.S. hardback edition, with corrections of errors pointed out by helpful readers, and some modest stylistic improvements.
The book can be ordered from online booksellers like Amazon and Barnes & Noble, and also from Powell's Books, or via Booksense.