Unknown Quantity was published in hardback in May 2006 by Joseph Henry Press of Washington D.C. Various other editions and translations have subsequently been published.
The book is a history of algebra, aimed at a general (i.e. not particularly mathematical) readership. It is a historical survey, with just as much math as the reader will need in order to follow the main lines of development.
There is a general description of the book here, with the table of contents and a list of the most important mathematicians covered in the text.