Singer does an excellent job of providing a sufficiently detailed review of the private military industry (nee mercenary sector) in this book. He starts off with the historical background and the cyclicality of their use and legitimacy over the millenia (with a focus on the last 8 centuries), to then progress to the reemergence of the sector in the late 1980s / early 1990s.
Subsequently the reasons and enablers of a return of the private military enterprise are examined and he also presents a couple of case studies on representative firms from each sector. Subsequently he also examines implications and possible future developments. While he does attempt to construct some theoretical frameworks around it, those are but a small part of the book, and while they arguably do not add much to the quality of the book, they definitely do not make it a dry, academic publication.
Where Singer scores very highly in my opinion is in the analysis - all aspects are treated in sufficient depth to make their inclusion worthwhile and he presents a very even handed picture, without passing judgement either way. The book is not a piece of prescriptive writing, it allows the reader to form their own opinion, based on the very well researched and documented analysis of the sector.
As the other review mentions, one of the few downfalls is the occasionally grave oversight in spelling, grammar and the odd erroneous weapons system mentioned - however these occur on very few occasions and it seems like the odd page was completely overlooked in the editing process rather than these errors cropping up all over the book.
All in all a commendable performance, in light of other, rather lightweight books passing for analytical work these days.