Ian Kershaw is one of the foremost English-language historians of Nazi Germany. His two-volume biography of Hitler will probably be the standard work on the subject for the next couple of decades. "The Nazi Dictatorship" is being marketed partly as an "introduction" to the history of the Third Reich. This is misleading, as it implies that this is a useful book for someone who doesn't know anything about the subject. It's a brilliant and enlightening book but it does demand that the reader is already reasonably familiar with the contours of the subject, because what Kershaw is doing is outlining recent tendencies in the writing of Third Reich history. It's no good reading about the intentionalism vs. functionalism debate if you don't really know who Hitler was or what he did.
What is the "intentionalism vs. functionalism debate"? Broadly speaking, it's an ongoing dispute between two schools of historians about how power was exercised in the Third Reich, and in particular how, for example, the Final Solution came about.
The extreme version of the intentionalist argument about the Final Solution is that Hitler had a definite plan to exterminate all the European Jews from fairly early on, and that he built up the Nazi Party and dragged Germany into a war because his most important aim was not to secure more territory for Germany or to revive the German economy or to uplift the morale of the German people, but to kill the European Jews. This is the view most famously expounded in Lucy Dawidowicz's very popular book "The War Against the Jews 1933-1945". It gains its support from Hitler's very explicit hatred of Jews, and his frequent public comments that he was in favour of "getting rid" of them or killing many of them, one way or the other. It also appeals on an emotional level; after all, one of the most humanly significant consequences of the rise of the Third Reich was the mass murder of millions of European Jews.
Against this, there is the functionalist argument. The functionalists argue that although Hitler undoubtedly hated Jews, neither he nor any other member of the Nazi leadership had plans to exterminate them until well into the Second World War. The main strength of the functionalist argument is that it has much better support from the historical evidence. The functionalists are not Holocaust deniers: they do not argue that the Nazis did not kill millions of people. They just argue that the Final Solution came about for a number of reasons, of which Hitler's hatred of the Jews was merely a very important one. The historical record shows that the Nazis experimented with a variety of methods of "getting rid of" the Jews, including trying to expel them into Soviet territory and even flirting briefly with the idea of establishing a colony for them on the African island of Madagascar. But almost from the very beginning of the war, German death squads were executing Jews by shooting, and it was only a matter of time before murder came to seem the simplest and most efficient method of "getting rid of" Jews. The gas chambers and extermination camps were developed because the Germans thought that shooting innocent people was wrong - not because they thought it was too cruel to the victims, but because it was too hard on the death squads.
There is much to be said for the functionalist argument, but in its extreme form it can amount to the suggestion that the Final Solution was almost a colossal muddle-through, which hardly fits what we know about what Hitler wanted to happen. Kershaw, and other historians like him, proposes a synthesis between the two approaches. In Kershaw's view, Hitler deliberately refrained as much as possible from making actual decisions. He exercised his (unquestioned) power by giving his consent to plans and schemes developed by his subordinates, who were usually trying to come up with something that they thought would please him. All Hitler had to do was make public threats about what was coming to the Jews; his subordinates would correctly interpret this as a hint that anybody who wanted to persecute the Jews would probably not be punished for it. Joseph Goebbels confirmed this picture of Hitler's complicity with the Final Solution when he noted in his diary that when it came to the "Jewish question", Hitler was always in favour of the most extreme and radical solution. No wonder that mass murder, rather than forced emigration, came to be adopted as official policy.
I thoroughly recommend this book to serious students of Nazi Germany. For casual readers, it's hard to think of a brief introduction to the subject which is also up to date with current research. Richard J. Evans' brilliant three-volume history of the Third Reich is a great work, but a bit of an investment. A better place to start might be the recent one-volume abridgment of Kershaw's "Hitler".