Os Guinness, The Call: Finding and Fulfilling the Central Purpose of Your Life (Word 1998).
This book is divided into 26 chapters for easy use as a devotional guide.
From the publisher (1998 edition): Os Guinness has penned a classic reflective work on life's purpose. Far bigger than our jobs and accomplishments and higher than our wildest ideas of self-fulfillment, our calling does more than give purpose and meaning to our lives--it completes God's plan for us.
Note: An updated edition was published in 2003 that includes a workbook for individual or group study.