It would be nearly impossible to overstate the importance of this book. It is, I believe, more pivotal in shaping the American popular understanding of Nazi Germany than any other book ever published. As such, it has helped shape everything from representations of Nazis and their victims in motion pictures to media protrayals of accused war criminals living in the United States.
As a work history, this book is also extremely impressive. Shirer makes extensive and critical use of a plethora of primary sources, including captured German documents and testimony from the Nuremburg trials, and this gives his account considerable credibility. His writing style is engrossing, making the length of the book seem less gargantuan than it is. I doubt that I would be able to identify a more comprehensive or readable single-volume history of Nazi Germany.
It should be understood, however, that Shirer does not really intend for this book to be merely a history of Nazi Germany. It is a morality tale. Shirer is aghast at the destruction and barbarity that Nazi Germany wrought in the world, and this book reads like an indictment of everybody everwhere who had a hand in allowing the barbarity to occur. Nobody can escape responsibility, not common Germans who brought Hitler to power, not the German generals who were unwilling or unable to control Hitler, not the German businessmen who profited through Hitler's various barbarities, not the Anglo-French architects of appeasement, and most of all not the Nazis themselves.
Of course, Shirer's sense of moral outrage sometimes causes some unfortunate lapses. It is rare that Shirer does not call Goering fat when Goering pops up in the narrative. Similarly, he invariably uses "fatuous" to describe Ribbentrop and reminds us on numerous occasions that Rosenberg was a "dolt." I have no idea what Goering's girth has to do with anything, and Shirer never really gives us a real idea of why he thinks that Ribbentrop was fatuous or Rosenberg was any stupider than any other member of the Nazi elite. Gratuitous pejoratives are distracting and unfair.
And then there's the matter of Ernst Roehm, Hitler's chief of the SA. Roehm and the rest of the members of the SA were a bunch of terrorist thugs who got votes for the Nazis by intimidating the opposition, but to Shirer, this thuggery is eclipsed by the fact that Roehm and some other of the SA leaders were or were thought to be gay (which Shirer consistently refers to as a "perversion"). To say the least, the credibility of Shirer's moral outrage at the racist and anti-semitic doctrine of the Nazi party is undermined by his bald homophobia.
More than that, Shirer makes no real attempt to understand why the British and the French behaved as they did in appeasing Hitler. He ascribes it to some sort of moral failing, and while this may be the case, it is only part of the story. France and Great Britain were bankrupted by the Depression. They couldn't really afford to rearm, and they were desparate to avoid a war at least partly out of a misplaced fiscal restraint. This fact does not obscure the reality that the appeasement policy was short-sighted and stupid, but at least it makes the whole thing more comprehensible. Likewise, Shirer doesn't really understand that Germany's rearmament was paid for with checks that the Reich couldn't cash without plunder. By 1939, the German economy was a house of cards that was about to collapse without a capital infusion. Unfortunately, one wouldn't know that from reading Shirer.
Finally, the emphasis that Shirer puts on different periods of the Third Reich is disproportionate. The war years, especially from 1943 to 1945, are sped through with very little detail about anything except the various plots against Hitler. It's almost as if Shirer ran out of gas after 800 pages or so. It is admirable that Shirer does not get bogged down the military details of the war, but at the same time, I would think that the war years deserve more than 25 or 30% of the book.
By all means, read this book, especially if you have only cursory familiarity with Nazi Germany. It is generally well-written, accessible, and reasonably comprehensive. Just beware of the problems with it as you are reading.