I just finished the book the Lost Art of Gratitude by Alexander McCall Smith, and while it isn't about Christianity or any religion in particular, it does cover the virtue.  Sometimes  librarians have grateful patrons.
Once or twice a student has come in and said "you helped me get an A on my paper," or words to that effect. However, is an expressed gratitude how we know we've done our job? 
I believe Anne Lamott said she has two prayers: Help me, Help me, Help me, and Thank you,Thank you, Thank you.   When I broke my wrist which is healed by the way, those were definitely the essence of my prayers!