Just to give things some scale, the longest book series in WH 40k is the hours heresy. It’s 60+ books. And that takes place in the 30th millennium, so it’s just background to current times lore.
There are more than 300 novels to read, and that’s just counting novels. There are also codexes (codecies?), other supplementary books, other game books (like dark heresy), and the many issues of the white dwarf magazine.