At least over here, when you go to any military section - which is found just about on any book store (new or 2nd hand), is most of all WWII, and even further, most of all focus on germany.
I can't really recommend any specific books in English, but I think good way to start, would be when idealism of national socialism was fresh and untainted. I would guess in every european country, there was NS books published prior to WWII. In Finland from the biggest publishers a'la WSOY, Otava etc, but also smaller printers. Good choices could be Kansan Työ (1936), Kansan Valta (1936) both with neat finnish style swastikas on cover.
Perhaps also 1941 Kansallissosialismi Maailmanhistoriallisen kehityksen tuloksena by judge Jussi Leino, published by Otava. National Socialism as product of development of world history.. perhaps poor translation, but those who are bored to read about atrocities and war, can read about the actual ideas and spirit of the movement or the masterminds who created & introduced the ideas. Of course, related in times long gone, but most interestingly, in times when none of these movements had really been put in practice long enough to really see them in negative light.
These books would be more of philosophy and political agenda. Criticizing alternatives, and proving how their route is the only logical way to go.
