I currently seem to be drawn to books about the world wars. Diary of a man in despair came out in English translation in 1974. I became aware if it reading Richard Evans trilogy on the whole Nazi movement. It's taking me a little while to get round to reading it. Friedrich Malleczewen was a very cultured writer who the Nazis eventually caught up with and killed even though he was a minor character in every sense and no agitator. He kept a secret diary through the year wars and before which is a fascinating and well written read. Not sensationalist in any way it gives a good idea of the horror of those years. Malleczewen's Catholic faith peeks through at certain times. It is difficult to see how anyone can look at those years with any degree of composure without a theistic worldview. Always the remarkable thing for many of us is that it happened here in Europe and not so very long ago. How sobering.
Bonhoeffer uses a similar phrase 'worldly Christianity'. It's J Gresham Machen that I want to line up most closely with. See his Christianity and culture here. Having done commentaries on Proverbs (Heavenly Wisdom) and Song of Songs (Heavenly Love), a matching title for Ecclesiastes would be Heavenly Worldliness. For my stance on worldliness, see 3 posts here.