I’ve read a few of the HH books, but it’s intimidating to get into it now. So much there.
Thought The Thousands Sons was one of the better sci-fi books I’ve read, even outside of 40K books. Getting a bit away from HH, I liked The Carrion Throne, too, and the Farsight series has been a good look at the Tau.
Read the first few HH books a few years ago. At that time, I wasn’t much convince about Horus’ turn. It seemed to be too sudden: for the first book and a half, he appeared very loyal to his cause, then he gets some kind of illness, has a vision or something like that, and then becomes the traitor we all know and love/hate. But it has been a long time since I read them, and I was still very new to 40K at that time, so maybe I’d see it different if I read them again.