World War Two In An Hour is a useful short history of WW2, costs $2.99 at Smashwords. It obviously doesn't go into a great deal of detail, but it's a good overview.
Inside the Third Reich was written by Albert Speer, Hitler's armaments minister, while he was in prison after the war. Free from the Internet Archive.
Those two are the only factual e-books that I've read on either war. It's worth having a browse around
Project Gutenberg and the
Internet Archive. WW1 is long enough ago that there are quite a few books about it that are now out of copyright, and the Internet Archive has some books about WW2