Not hard Sci Fi at all but I'm about 15 books into the Horace Heresy novels and while some are better than others they are all consistently enjoyable. The first 3 are particularly strong. I think my favorite so far is Legion which is a spy thriller but with all of the over-the-top ridiculousness that 40k is known for.

edit: there are approximately a billion of them https://wh40k.lexicanum.com/wiki/Horus_Heresy_Series

Not hard Sci Fi at all but I'm about 15 books into the Horace Heresy novels and while some are better than others they are all consistently enjoyable. The first 3 are particularly strong. I think my favorite so far is Legion which is a spy thriller but with all of the over-the-top ridiculousness that 40k is known for.