Guards! Guards! (Discworld, #8)

by Terry Pratchett

The Supreme Grand Master listened to this with a slightly light-headed feeling. It was as if he’d known that there were such things as avalanches, but had never dreamed when he dropped the little snowball on top of the mountain that it could lead to such astonishing results.

The relevant equation is: Knowledge = power = energy = matter = mass; a good bookshop is just a genteel Black Hole that knows how to read.