•Why it earns a slot
One of the first serious attempts to study religion empirically rather than theologically or dismissively, and the source of the still-used 'once-born versus twice-born' framework for religious temperament.
James, a psychologist and philosopher, set out to study religion the way a scientist studies any other human experience: not by asking whether God exists, but by examining what religious experience actually does to the people who have it. His method was to judge beliefs by their practical fruits rather than their psychological or physiological origins.
This distillation is written from the freely available original, which is always the better read when you have the time: Project Gutenberg.