Like other Osprey books, this is small, but packs quite a lot for its size. The most informative, to me, part was the analysis of how the armies ended up he way they did and why the Poles had a cordon defensive strategy. Particularly useful was the point about Germany having an economy considerably bigger than that of Poland. When you are richer and more industrialized, it's easier to build trucks and tanks while the poorer country is more dependent on cavalry (incidentally, the book does a good job in exploding the canard that the Polish reliance on cavalry was due to obsolete doctrine). Although the Polish campaign is seen as the dawn of blitzkrieg warfare, the book points out that much of the German success was due to having considerably more artillery. That doesn't mean that Germany didn't make better doctrinal choices. Although the Poles had a tank that was qualitatively equal to the German Mark II, they still deployed them in penny packets for infantry support while Germany formed them into Panzer and Light divisions. The greater German commitment to radio is explained and it had much to do with greater German ability to react to developments and shape events.
The book does a good job with the campaign. Two key points were the Polish defense strategy and their delayed mobilization. The latter was a consequence of Anglo-French pressure, which view Polish mobilization as being potentially provocative and thus discourage it. This encourage a forward defense strategy by Poland since many of the population centers from which the reserves would be drawn were close to the German frontier. This strategy was also to discourage German "salami tactics", where the Germans would perhaps simply size a few border areas (a la Sudetenland). One nice touch is it's addressing the myth that Polish cavalry charged German armored vehicles (the action that gave rise to that myth is explained). The rest is a breakdown of the initial battles, the breakthroughs and the siege of Warsaw. The Soviet invasion, although anti-climatic in some respects, isn't stinted.
The maps and photos are up to the usual Osprey standard. A very useful, for its brevity, book on the first campaign of WWII.