Hitler's secretary towards the end of the war wrote in 1945 that Hitler had told him.
"September 1938, that was the most favourable moment, where an attack carried the lowest risk for us... Great Britain and France, surprised by the speed of our attack, would have done nothing, all the more since we had world opinion on our side... we could have settled the remaining territorial questions in Eastern Europe and the Balkans without fearing intervention from the Anglo-French Powers... We ourselves would have won the necessary time for our own moral and material rearmament and a second world war, even if it was altogether unavoidable, would have been postponed for years." ^(1)
The important difference between the Munich Agreement and the invasion of Poland is that while Germany certainly could have easily beaten France militarily in 1938, in 1939 the rearmament programmes of France and Britain had improved their forces greatly in comparison and the source relies strongly on hindsight assuming that the collapse of France would have occurred on the same lines while ignoring the time it would take to completely defeat both Czechoslovakia and Poland as the secret Pact with the USSR wasn't signed until 1939, so the Soviet Union's role in the collapse of Poland would have been uncertain. Furthermore, Hitler broke the Munich Agreement almost the second he signed it with first undermining Czechoslovakian independence leading onto the First Vienna Award and with the German seizure of the rest he helped ensured that the Allied powers became more determined in opposing him in the future which led directly to the British guaranteeing Poland's independence before the Danzig crisis.
A good book that helps cover this period is The Road to War, Richard Overy. He breaks down his chapters by major powers and runs up their domestic and political positions up until they enter the war. Hope this helps.
^(1) Hitlers politisches Testament: die Borman Diktate Vom Februar und April 1945 ( Hamburg, 1981), pp.100-101, entry for 21 February 1945