Tenebrae
Only my second Argento, somehow, after (obviously) Suspiria. An American novelist, Peter Neal, comes to Rome to promote his newest book but slightly before he gets there (thus removing suspicion from him at least) a woman is murdered and pages from his novel is stuffed down her throat. To make the point even further, Neal gets a letter saying yes, his books are the reason.
A bunch more murders happen. There’s a girl in what seems to be a flashback - unrelated to everything else - who rejects an unnamed boy from a beach orgy. The guy who clearly did it is killed himself, but the murders continue. The police woman gets murdered! Neal’s assistant Gianni is the only person who seems upset by any of this in a vaguely realistic fashion.
But the most shocking thing of all is that in the end, it all pretty much makes sense and is actually pretty clever. Good job, everyone. Not as good as Suspiria though.