In this article, Stuart McAllister of Ravi Zacharias International Ministries writes how the book of Ecclesiastes is an essential resource to ponder questions such as: What is the nature of reality? What is it (existence) all about? What is the purpose of life (if any), and to what should we try to give answers?
In summary, the article provides five "lessons" in describing the Ecclesiastes apologetic approach:
Lesson One: The Art of Effective Diagnosis
Lesson Two: The Limits of Reason, Rationality, and Analysis
Lesson Three: Life from an Earthly Perspective Often Seems Pointless
Lesson Four: The Necessity of an Answer
Lesson Five: Accept and face the limits of knowledge
You can find the article here.