Big agree about Harrow the Ninth. It is a very challenging read and if you liked the series enough I strongly suggest to re-read Harrow. Really all of them. The Locked Tomb series is a mystery/horror series where the last 1/3-1/4 reveals most of the mysteries you could have puzzled out by reading it. So they are a lot of fun to re-read and catch all the clues you missed the first time. Especially with the knowledge of what happens later (Book 3: Nona really explains a lot of world-building stuff that makes the earlier setting make more sense. Did you know cows look at the sunset?)
But Harrow is another beast in this vein. For anyone reading it for the first time or wanting to: Harrow is told from an extremely unreliable narrator who is processing immense trauma and incredibly esoteric magic. Not all of the events are happening at the time the book suggests (flash backs and forwards are not called out) and the narrator doesn’t always know either. The end really ties it all together with a coffin lid.