Oswald Chambers' My Utmost For His Highest is one of the most popular devotionals ever complied, and a favorite of mine that has never ceased to kick me in the gut when I'm needing it or feeling a little stale. One of the entries that has always jazzed me up is "Called By God" which looks at Isaiah 6:8, one of my favorite sections in Scripture, and one that's often used for people's calls to ministry.