Some time ago I downloaded a copy of Sinclair Ferguson's In Christ alone partly because it was free. I have been reading its nearly fifty chapters mostly one by one and have now completed it. It is a wide ranging book full of rich doctrine and good practical advice. It is well written and full of gems. Most of it reads like magazine articles for style and it would be an ideal devotional aid for use day by day or as a Sunday afternoon reader. What a blessing to the church Sinclair Ferguson is. I notice he is speaking in London in the summer, at the Proclamation Trust meetings.
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.