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.