Thousand Sons was easily one of my favorite HH books, and pairs exceptionally well with Burning of Prospero. I think I finished Thousand Sons in like 2 sittings.
Given that, you may not enjoy the same books I like, but here's a list of what I think are the best HH books. No particular order, and none from the Siege, which is universally excellent.
- Legion
- Know No Fear
- Vengeful Spirit
- Nemesis
- The First Heretic
- Betrayer
- Tallarn series of novellas
- Pharos
- Slaves to Darkness
Also not to argue with you but the Thousand Sons thinking they're all the smartest people in the galaxy but they're really just Space Marines with their heads up their asses is a core element of who they are. It's a genetic inheritance from Magnus and is central to his character.
It's fine that you didn't like the book, and not trying to argue or anything, but that disconnect may be why you didn't like it so much. Every Thousand Son is an unreliable narrator who thinks they kick ass at everything.