Geraldโs Game by Stephen King โ his scariest book (for me) and the ending still haunts me. 10/10, big content warnings. It by him is also really great, tis 1000 pages though.
The Island by Adrian McKinty is a fabulous murder thriller that actually had my heart racing.
Blood Sugar by Sascha Rothchild is not Scary per se, but really well written. The narrator is a murderer accused of a murder she didnโt commit!
Kill for Love by Laura Picklesimer is not scary, but plenty of gore.