My definition of a great stalker book is one where the threat feels so ordinary it becomes unnerving. A lot of stories go for the over-the-top, hyper-competent villain, which can be fun but often loses that crucial element of psychological realism. I'm much more unsettled by narratives where the stalking weaponizes mundane social structures—persistent but polite emails, 'coincidental' social media likes, knowing which coffee shop you go to because you checked in once.
Tana French isn't typically shelved in the thriller section, but 'The Witch Elm' does something brilliant with this. The violation isn't always a stranger in the bushes; it's the slow, insidious realization that someone you've allowed into your personal orbit has been meticulously cataloging your vulnerabilities. The suspense comes from the protagonist's own deteriorating grip on reality and memory, making you question every friendly interaction.
For a deep cut, try 'The Last Time I Lied' by Riley Sager. While the central mystery is about a disappearance, the framework is all about being watched and manipulated within a closed, competitive environment. The stalker's power isn't physical dominance but information—knowing secrets and using them to orchestrate paranoia. That, to me, is the peak of the genre: the fear that your own life is being used as a script for someone else's twisted play.