Courage and Calling: Embracing Your God-Given Potential, Gordon T. Smith.
This is my favorite all-around book on the subject.  Smith helps us understand how our giftedness, our weakness, our life in community and our day to day activities all contribute to an understanding of our calling.  He also has excellent chapters on the contexts of our calling: how callings change through the different stages of life; the role of suffering and moral challenge; working within organizations, and the need for solitude and community as we live out our callings.

The Call: Finding and Fulfilling the Central Purpose of Your Life, Os Guinness.
Os Guinness was the first person I ever heard speak on this subject. Divided into 26 chapters, the book contains lots of wisdom and the format helps you digest it slowly. The newest edition also contains a study guide.

Make College Count: A Faithful Guide to Life and Learning,  Derek Melleby.
Many young people go off to college without thinking about why or how it relates to their Christian life. This book is without a doubt the best book for students heading off to college; many have found it helpful even once they get to school.  It is a work of graceful substance, small enough to be read, and serious enough to be truly helpful.  So few students have a book that they will read that will help them grapple with the biggest questions of who they are and why they are in college. Highly recommended.

You can also listen to my sermon on the subject here.