Also called The Hollow Man, by John Dickson Carr.
I’m not saying I’ll never read another Carr mystery, but it’s possible. Carr’s plots are ridiculously complicated. I read this because it was a classic locked room mystery, but I tend to think locked room mysteries can be over-focused on the how of it all, and leave an unsatisfying story. The solution was not unsatisfying, I didn’t guess it and I bought it, despite the fact that blood only seemed to flow when and where the author desired it and it was absurdly complicated.
Perhaps the problem comes from the characters, none of whom seems normal. And there are very odd moments. Supposedly the whole house is on alert for the killer who has announced his arrival some think at 10:00. When someone arrives at 9:45, no one seems to think it could be the killer because he’s early. Are murderers all perfectly punctual? That means I’ll never become a murderer, thank goodness. So two people who are supposedly watching out are playing cards in the front room with the radio on and the door shut. Way to watch out, guys.
I kept reading this, despite the constant irritation that is Dr. Gideon Fell, because I did want to know what happens and also cover a Vintage Golden Bingo square. If you like golden age mysteries, you might want to skip Dr. Fell’s lecture on locked room stories late in the book. He has no compunction about revealing the solutions. He’s really quite a jerk. Fortunately, I’ve already read the Yellow Room and can’t remember the rest of them. The lecture could easily have been done without associating particular solutions with particular books, but I think Carr just wanted to show off how many he’d read and ruin them for those who hadn’t, which makes him quite an… but this is a family blog, so I won’t say what I’m thinking. He also includes a spoiler or two for his own work, Death-Watch. I hate that.