Between the First World War and the Second World War, there was another conflict that greatly impacted Europe. This was the Spanish Civil War. Although technically this was just a civil war between the Republicans and Nationalists, unofficially many of the nations that participated in the Second World War, were involved in the Spanish Civil War, at least unofficially. Hitler especially took advantage of this opportunity to perfect the strategies that he would use in the Second World War. You can find more episodes here.
My recommendation for an audio book is Hell and Good Company: The Spanish Civil War and the World It Made by Richard Rhodes. From the Pulitzer Prize-winning and best-selling author of The Making of the Atomic Bomb, the remarkable story of the Spanish Civil War through the eyes of the reporters, writers, artists, doctors, and nurses who witnessed it. Get this audiobook for free with a free trial of Audible through this LINK. Doing so helps support this podcast.