The previous two reviews have described the details fairly well. Watt provides an engaging account of Poland's birth amid a series of wars in the aftermath of WWI, its attempts at parliamentary democracy up until 1926 (when governments replaced each other with alarming regularity), Piłsudskis coup and subsequent dictatorship until his death in 1935 ("authoritarian but not fascist", is Watt's summary), and then the "government of the colonels" up until the destruction of Poland in 1939.
He is sympathetic to Poland and its plight, and the gargantuan tasks facing it on its recreation in 1918, but does not hide from its failings either. His basic viewpoint is that interwar Poland achieved much internally in difficult circumstances (for example in literacy and in land reform), but pursued a foreign policy that was overly aggressive, often unrealistic and generally unwise. Dictator Piłsudski gets off relatively lightly, while Colonel Beck, who was responsible for foreign policy 1935-9, is roundly criticised.
Given that Poland and its fate was a major feature of the Versailles settlement at the end of WWI, and of course the scene of the start of WWII when it was attacked by Nazi Germany and Soviet Russia, the struggles of this short-lived state is a subject that ought to be better understood than it generally is in the West. This book is an excellent place to start, as it gives a credible and readable account that is free from an obvious axe to grind.