The question should be rephrased, to make it not that obvious and more interesting what the answer will be: what is the most influential non-religious book?

I guess that’s hard to determine, which is the most influential. I think religious books like the Bible, the Quran, … were very influential. Or “Capital” by Karl Marx.

I think Gutenberg’s Bible, because the translation destroyed the privilege of sole interpretation by the clergy. From a technical perspective, it was the cornerstone of the information age through the invention of moveable letters/ letterpress.