While the book does a great job of keeping you guessing throughout, the clues pointing to David as the stalker are:
* His obsessive behavior: He's shown to be possessive and controlling, even going so far as to follow Anne and attempt to manipulate her friends.
* His knowledge of her life: He's aware of details about Anne's life, including her work and her home address, which suggests he's been keeping tabs on her.
* His sudden reappearance: He resurfaces after a period of silence, coinciding with the escalation of the stalking.
* The phone calls: The calls Anne receives are often about subjects that only David would know.
It's important to note that *Tightrope* is a psychological thriller, and the suspense is built around the uncertainty of the stalker's identity. While the clues point to David, the book keeps you guessing until the final reveal.