The first place any review of this short book would need to start out at is the author himself, David Nicolle. Dr. Nicolle is one of the world's leading scholars of medieval warfare, both Western and Islamic. He has published dozens of books on the subject. He is an impeccable authority on the topic and this book shows it.
The book follows the typical Osprey format for the "campaign" series, starting off with a brief introduction to the two sides in every respect (i.e., leaders, armies, tactics, strengths and weaknesses, experience levels, etc.). The book then goes on to elaborate on the geopolitical situation of each side, basically that the Christians were very much mistrustful and resentful of each other and not very united in terms of command structure while the Ottoman state was very hierarchical and unified. This was a major factor contributing to the Ottoman victory.
Nicolle then follows the routes of the two armies before they even met, the battle itself and the immediate consequences. He elaborates, quite well, on the poor tactics used by the crusading armies relative to the tactical flexibility of the Ottoman forces, another major contributing factor to the Christian defeat. He also elaborates on the arrogance of the French forces, along with their lack of experience against Islamic military tactics, both in absolute terms and relative to their Wallachian allies who were much more suited to battle the Ottomans because of their greater tactical flexibility and considerable experience fighting the Ottomans in the past. It was this French arrogance in ignoring the advice of their Wallachian allies that also contributed, in a major fashion, to the Christian defeat. Nicolle not only covers these major themes well but also covers a number of minor events quite well. For example, he provides numerous competing views as to what happened to many of the Christians captured during the battle.
Nicolle concludes the book with the immediate consequences of the battle, basically that the Christian alliance was shattered, the Christian states bordering the Ottoman state were seriously weakened and the Ottoman state consolidated its hold in the Balkans. All this eventually enabled the Ottomans to launch a long term successful war of conquest of the Balkans.
For the book's short length, 96 pages about half of which are illustration, the book does an excellent job at providing a succinct history of the battle for those with only an hour or two to spend on the subject.