There are already lots of reviews which focus on the formidable research evident on every page of this book and the attention given to the Somali perspective. As an enormous fan of the film I want to concentrate on its relationship with this book.
As I'm sure is the case with most people, I watched the film first and then read the book. What I really liked was the way in which relatively minor instances in the film are fully fleshed out and and contextualised in the book. For instance, Super Six One clips a building on its way down in the film and Bowden managed to interview one of the boys who lived in that very house. Accounts of the 160th SOAR pilots' phenomenal skills and superhuman confidence underline just how much of a shock it was for two helicopters to be grounded in battle. The tension between the Rangers and the "D-Boys" (Delta Force), largely acted out by Sanderson and Steele in the film, is given greater attention as Bowden focuses on the Rangers' admiration for Delta but reveals that some Delta soldiers did not have total confidence in the Rangers. The part in the film which sees the asthmatic Ranger almost deciding not to go back into battle is explored in the book and we see that Struecker, who asked him to reconsider, actually understood - and almost agreed with - his initial decision to stay at base, but he had to remain wholly professional. Sizemore's devotion, which caused him to put a knife to his own cast so that he could join the battle, is also revealed as true to life.
If I had a criticism of the book it would be one that is, in a strange way, also praise. The book manages to recreate the dizzying atmosphere of the film, in which (on a first viewing) we can quickly lose track of names and locations. The book is told in roughly chronogical order but it digresses so often into, say, a certain soldier's background that there are only a few moments where we get the same sense of tension and thrust as the film. But this is down to the sheer amount of research Bowden has carried out and the number of individual stories he wants to tell - stories which the film often glosses over or omits completely.