Using the Bible could be interpreted as ‘this is one of humanitys most important works so let’s preserve that first’ Yet the majority of people on planet earth are not Christian.
I’ll go with the Gutenberg homage though
The Tanakh also has the distinction of being one of the few works of literature that has survived for 3000 years largely intact, and I think it's considerably longer than the older parts of the Egyptian Book of the Dead.
The first reference quite explicitly says "number of copies each book sold over the last 50 years."
Sales is an approximation for readership. The second source also claims "figures may incorporate populations of secular/nominal adherents".
So, while I agree that populations that identify with Christianity may be larger, it's pretty obvious a lot of those do not observe the rituals the religions they identify with specify.
To get a better readership proxy, perhaps, one could use the share in sales of religious texts across the different demographics.
Then we need to also define what "read" means. Does it mean the book was read cover to cover or does it mean the book is read from occasionally? Religious texts are used in very different ways than other forms of literature, to the point comparison borders the questionable.