The book, says Worthington, is "based on an exhaustive analysis of 8,000 pages of documents released by the Pentagon (plus other sources)." It includes a narrative of the establishment and operation of the facility, a chronology of prisoner seizures, and the detailed stories of about 450 of the prisoner, as well as contextual/circumstantial information on the rest.
Note: This is not a review -- I haven't read the book yet. It sounds fascinating, though, and is probably the most detailed and factually honest account of Gitmo we're going to get until and unless Gitmo produces a Solzhenitsyn.