A Persian riddle

From The Spectator, 17 March 1990.

The Riddle
by Naser Khosrow (1003-1088)
Translated from the Persian by Dick Davis

I have a friend who, when I am alone,
Sits with me — and how intimate we've grown!
He talks, but what he says he never hears,
He is unfeeling, but he dries my tears.
He has one back, he has a hundred faces
As lovely as the spring in desert places
(Sometimes I thump him on the back — I must,
He gets half-smothered in thick, choking dust).
He talks, but soundlessly; he has to find
A clever man before he'll speak his mind.
Whenever I encounter him, his eyes
Recall the precepts of the good and wise,
And yet he's quiet till I look his way
Unlike some fools, who blather on all day.
In darkness he falls silent — which is right,
He is a Prince who glories in the Light.

The Answer is 'A Book'.