Very nice review here. Any better and I would say you needn't bother buying the book. About the equation, it is indeed a simplification of the full model -- trying to balance completeness with making sure it is understandable. As the book (and for those super keen, Temporal Motivation Theory described in my Academy of Management Review article "Integrating Theories of Motivation"), we add a constant in the denomenator to prevent the entire thing sky rocketing to infinity when delay approaches zero (in joke, one of the characters has a kid named Constance in reference to this).