A thought-provoking and helpful article here by Gilbert Meilaender, Professor of Theology at Valparaiso University: Vocation: Divine Summons. (Originally published in the Nov. 1, 2000 issue of Christian Century).
Meilander draws from Virgil and Augustine and Dante and Herbert and Lewis to look at the the meaning and significance of God's summons upon our lives and our vocation.
Meilaender's article was recently cited in an article by John Ortberg in Leadership Journal. Ortberg's piece, God's Call Waiting, is also worth reading.