I picked up 'Demon's Letter' because the cover looked cool, honestly. I kept waiting for some big supernatural reveal, but the main mystery is way more grounded and kind of sad. It's basically this woman trying to figure out why her reclusive, genius uncle, who she was never close to, left her his massive, bizarre estate and a bunch of cryptic notes instead of his direct family. The letters referenced in the title aren't from a demon; they're these coded business ledgers and personal journals hinting at a decades-old family betrayal and a hidden illegitimate child. The mystery isn't really 'what demon is coming,' it's 'what secret was so awful it tore this family apart and made this brilliant man live like a hermit.'
It's a slow burn. You're piecing together the past alongside the main character, and the real twist is that the 'demon' is just... human greed and resentment. I liked it, but if you go in expecting a paranormal thriller, you'll be disappointed. The payoff is more of a quiet, melancholic understanding than a shocking monster reveal.