I've been reading 40k since the first Horus Heresy novels dropped. Absolutely love all the Siege of Terra and Heresy novels, few fillers but I've really enjoyed them.

Got a lot of the other books as well. Ravenor, Eisenhorn, Bequin, Cain, Inquisition War, Space Marines battles and they're all easy to read with different levels of quality.

Lately, said quality has been a bit... Lax? There's some in game lore that has made me want to stop collecting them due to Deux Ex level nonsense around the Gods of Chaos and it just bored the snot out of me reading it.

Give the ones named above a read. Be wary at just how much lore there really is mind!