Although I say it often enough, there are still only a few books that surprise me when it comes to the Second World War, especially the Eastern Front. In this case, the book I'd compare this work to would be David Stahel's "Operation Barbarossa and Germany's Defeat in the East." I'd recommend reading Müller's volume first and follow that up with Stahel's ever evolving look at the German incursion into the Soviet Union.
Taking a look at "Enemy in the East" there are a few important things the author attempted to do with this monograph. He wanted to look at the history of German military plans against the Soviet Union, especially in the period after Hitler took power and through 1941. Simultaneously, he wanted to see who actually did the planning as in the postwar period the majority of the German commanders who remained alive created a narrative that portrayed Hitler as being the sole guilty party in the genocidal campaign that was the German invasion of the Soviet Union. Answering the question of where the plans for the invasion of the Soviet Union originated from also means creating additional context in understanding not only the evolution of the Second World War but also the Holocaust. This means engaging with the old "intentionalists vs. functionalists" debate. The conclusion reached by the author is that it was Halder and the army that created the original plans for the invasion of the Soviet Union (which were not ideological, although perhaps a little too idealistic) but it was Hitler who created the genocidal form the war would take in the early periods of 1941 without much opposition from the army high command.
What the author does well, better than any interwar history that I've come across previously, is show the interplay between Hitler's Germany and Poland. That relationship receives a rather large amount of attention, while that of Germany and Great Britain, France, Japan, Italy, the Soviet Union, Romania, Czechoslovakia, etc., receive less attention but all play very vital roles in how Hitler's plans for the Second World War evolved. In some ways I think more attention should have been paid to the diplomatic relations between Germany and the Soviet Union as there are connections the author misses and would have added even more context to an already fascinating study.
According to Müller, Hitler had always planned to go to war with the Soviet Union and might have actually done it in 1939 with Poland's help if not for how certain events developed. Poland's move away from Germany toward Britain and France meant Germany could no longer continue creeping slowly east and utilizing additional eastern territories for an eventual invasion of the Soviet Union (with or without Poland's help). This meant an eventual deal with the Soviet Union had to be made. After invading Poland, and having Britain and France in turn declare war on Germany, Hitler once again had to alter his plans and invade France, in order to avoid a future war on two fronts. Only after the defeat of France was Hitler ready for a final showdown with the Soviet Union. However, German hubris, combined with unrealistic evaluations of both the Soviet Union and Red Army, resulted in clashes between Halder and Hitler and the eventual failure of Operation Barbarossa and Germany's eventual defeat.
Although this is very well researched work, there were some weaknesses amidst the many strengths. While there is no question the author is an expert in his relevant field(s) when it comes to Nazi Germany, Hitler, and the Wehrmacht, there were a few problems with how the Soviet Union, Stalin, the Red Army, etc., were portrayed. In general I think the author thinks quite highly of the Wehrmacht's capabilities, even before the invasion of Poland, and had a rather inadequate appraisal of the Red Army's abilities. Furthermore, the author compares the German invasion forces of some 3.7 million troops (German and allied) to Red Army forces in Soviet Western Military Districts, 2.9 million. The problem here is that western military districts stretched all the way to Moscow, Soviet forces on or near the border totaled only some 1 million men. Finally, one somewhat oft cited argument is that the Soviet counteroffensive outside Moscow was enabled thanks to Stalin siphoning off divisions from the Far East to face Germany in the west. Although this was done, it was a gradual process that began in the summer of 1941 and divisions from the Far East, while making a difference, were not decisive.
The above aside, this is a highly recommended text for those interested in the interwar period and the evolution of Hitler's plans for war and expansion throughout Europe. This is simply an indispensable volume that puts much of the interwar period and the lead up to the invasion of the Soviet Union in a new light.