Thanks for your recommendation to read Ian McEwan's Enduring Love. The protagonist, science writer Joe Rose, is relentlessly pursued (or so it appears) by a stalker, Jed Parry. Rose comes to the view that Parry is suffering from a rare form of erotic fixation called de Clerambault's syndrome. Or perhaps it's Rose who is delusional. Either way, it's breaking up his marriage.
One of McEwan's special skills is to illuminate the inner life of his characters. Several times, I had that jaw-dropping recognition that Joe Rose's thoughts, feelings and behaviours were exactly those which I would have had in his situation too*. Scary.
McEwan keeps us guessing even to the end. Was Enduring Love a novelisation of the real-life psychiatric case, reprinted as Appendix 1? Or did he make that up as well? One clue on the penultimate page settles it.
* Except for the illegally-acquired gun. I would have done that somewhat differently ...