“Treat books gently; for books are kind friends.
We benefit by their advice, and they exact no confessions.”

Most of this 100+ year old advice is still good! Some of my favorites are:

-“Books are not intended for card-racks or for
receptacles of botanical specimens.”

-“Books should not be toasted before a fire or be
converted into cushions to sit upon.“

-”Never destroy an original binding upon an old
volume if the binding be in tolerable condition.
An old book should not be rebound, unless it is
essential to its preservation ; and then it should be,
as far as possible, a restoration. “

Source: “ A manual of the art of bookbinding : containing full instructions in the different branches of forwarding, gilding, and finishing : also, the art of marbling book-edges and paper : the whole designed for the practical workman, the amateur, and the book-collector” by James B. Nicholson. Published 1878. 

Available for free on the internet archive. No known copyright restrictions.