I am now about 40% through the audio book of "The Rise and Fall of the Third Reich", by William L. Shirer. Before, I knew not much of World War II, and this (very) lengthy book seems comprehensive enough that after reading it I'll know more than enough about it.
The problem with the audio book form is that it challenged my memory of European geography a bit too much, and so now I have to look at a few maps from time to time. At least finding WWII maps online is not too much of a problem, unlike made up fantasy worlds like the Song of Ice and Fire by George R. R. Martin.