•Why it earns a slot
Why it earns a slot: it is the first book to make personal, contradictory, in-progress thought the whole subject, and Montaigne says so himself: he paints passage, not being.
Montaigne set out to describe one ordinary man completely and honestly, and in doing so invented the essay as a form for thinking out loud.
This distillation is written from the freely available original, which is always the better read when you have the time: Project Gutenberg.