My friend Aaron mentioned how he was thinking of work and calling in terms expressed in Simon Sinek's book Start With Why. He saw significant overlaps in Sinek's work with Keller's work in Every Good Endeavor. I think Aaron is on to something... if we align the why of our work with God's work, then we're on the right track.
I haven't yet read Sinek's book, but I found this TED talk in which he lays out the basis for his work. Take a few minutes and check it out.