The Modern Gothic Legacy of Peter Straub
I keep coming back to how Straub treated the supernatural as a kind of emotional weather. Ghost Story scares you through what the characters are willing to forget. The shapes themselves are secondary. That's a much harder trick to pull off than a jump scare, and it's probably why his influence shows up more in literary horror than in the splatter end of the shelf.
The Blue Rose trilogy is where I think he really locked in his signature move: taking a crime story and letting it rot into something mythic. Koko starts as a hunt for a war criminal and ends up somewhere far stranger, but you never feel the gears shifting. That seamless slide from realism into nightmare is the thing I'd want any aspiring horror writer to study.
I'd also argue Straub's collaboration with Stephen King on The Talisman matters more than people give it credit for. It proved two very different sensibilities could share a universe without canceling each other out. That kind of cross-pollination is rare, and it opened doors for the shared-world experiments we see everywhere now.
I don't think Straub ever got the mainstream cultural footprint of King or Barker. But the writers who love him tend to love him hard, and that's often the more meaningful kind of legacy.