“I think my great book is Born to Sing: An Interpretation and World Survey of Bird Song.”
In Herbert F. Vetter, " Not The Average Philosopher http://www.harvardsquarelibrary.org/unitarians/hartshorne.html", Harvard Magazine, May/June 1997, Volume 99, Number 5. Vetter was surprised by this, given Hartshorne's dozens of substantial books on theology.