We live in an area where there are many Muslims. Opportunities to speak are few and far between but I feel it is important to at least know something about Islam. James R White's book What every Christian should know about the Qur'an is probably a correct title as well as an eye catching one. He painstakingly works his way through the material writing about Muhammad and the supposed revelation of the Qur'an, what it says about the Trinity, the cross, salvation, the gospel and other matters such as supposed prophecies about Muhammad in the Bible and how the Qur'an reached its present form. He writes firmly but fairly and if any suitably educated Muslim would sit and read this book they would be forced to stop and consider many things. It is also very helpful to Christians in informing us on the things we need to know and showing where the argument needs to take place. This book is well worth getting hold of. For a proper review see Themelios 38:3.
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.