This "ancient book" is almost certainly an original creation of Banks. The use of the word "creat" to indicate a class of being (distinguishing biological from artificial, quick from slow, faring from planet-bound) appears to be an entirely novel neologism...
"It was a truism that there was not just one galaxy, there were many. Every variety of widely spread sentient life - plus a few creat categories which were arguably non-sentient though still capable of interstellar travel - and sometimes even every individual species-type tended to have one galaxy to itself. The Faring - a trans-category that covered all such beings able and willing to venture beyond their own immediate first-habitats -were like the citizens of a vast, fully three-dimensional but mostly empty city with multitudinous and varied travel systems. The majority of people were content to walk, and made their slow progress by way of an infinitude of quiet, effectively separate deserted streets, quiet parks, vacant lots, remains of wasteland and an entire unmapped network of paths, pavements, alleys, steps, ladders, wynds and snickets. They almost never encountered anybody en route, and when they got to where they were going, it would be somewhere very similar to the place they had departed from, whether that place had been a star's photosphere, a brown dwarf's surface, a gas-giant's atmosphere, a comet cloud or a region of interstellar space. Such species were generally called the Slow."
... and the term "fellow creats" which Banks uses earlier in the novel, but not in a quote, as if it's a common expression (which it obviously isn't, as your own Google research has shown)
The projection appeared to draw breath. 'But that is not the point. The smallest chance of the greatest reward, when so few may compete for it, makes the contention compulsory. All that matters is that we may have been presented with an extraordinary, utterly unprecedented opportunity. We would all be in serious, even ultimate dereliction of duty if we did not do everything in our power to seize that opportunity, not just on our own behalf but for the good of all our fellow creats, and for those generations yet unborn.'