"The kraken sleepeth."

 The kraken is a large, squid-like sea monster in Norwegian folklore. Alfred, Lord  Tennyson wrote the lyrical poem, The Kraken (1830), quoted here. As a ‘worshipper’ of  books, Aziraphale would be sure to recognise the reference.

The Kraken  by Alfred, Lord Tennyson

Below the thunders of the upper deep;

Far, far beneath in the abysmal sea,

His antient, dreamless, uninvaded sleep

The Kraken sleepeth: faintest sunlights flee

About his shadowy sides: above him swell

Huge sponges of millennial growth and height;

And far away into the sickly light,

From many a wondrous grot and secret cell

Unnumber'd and enormous polypi

Winnow with giant arms the slumbering green.

There hath he lain for ages and will lie

Battening upon huge seaworms in his sleep,

Until the latter fire shall heat the deep;

Then once by man and angels to be seen,

In roaring he shall rise and on the surface die.