You will be allowed a little light racism, just dont go too far.

I think this list is pretty close to the best books, you can jump off with series based on your favourite characters like Garro if they click for you: https://booksreadlisten.com/best-horus-heresy-books/

Couple those with the Siege of Terra but skip may be the first wall (as I detest Thorpes writing) and Mortis as its a bit boring.

People might say that way spoilers lye, however the ending was main lore even before the series started.

They didn’t expect the success of the first few that made the times best seller list, thats like massively more than they expected to sell, so they increased the planned number and the quality dropped massively as they expanded to the authors to suit.

If you read just the best books you’d still get the same favour but cut it to about 20 books or so, including the best ones in the Siege of Terra that are the actual ending.