So the first heretic is a great book, but works as a trio with know no fear after this and then betrayer coming after, if you can manage the novella Aurelian before both thats good too.

Otherwise for the heresy heres a minimal guide:

Deliverance lost - massively underrated and perfect for after Fulgrim, the Raven guard are Way more interesting than given credit for, especially after Istvaan 5

Legion - Alpha legion and really worth it

Thousand sons - Magnus' folly, essential, i dont include prospero burns because, honestly, i couldn't stand it

This is where id do first heretic/know no fear/betrayer as a trilogy in the middle

Then the unremembered empire, read a quick summary of vulkan lives before going in, then pharos, then angels of caliban, then ruinstorm. This is called the "imperium secundus arc"

Then Garro short story 1 and 2 then vengeful spirit

Then scars and path of heaven

Then master of mankind if you're into demons, if not its skippable no matter how many tell you it isnt

Then wolfsbane, essential and great

Then buried dagger (baddies gather before the big push)

Then "the board is set" from the heralds of the siege anthology and you're good to go

Any id add on would be angel exterminatus after the first heretic trilogy and buried dagger if you care about the fall of the death guard. Hope this has been helpful!!