Oblivion by Peter Abrahams

Peter Abrahams excels at exploring altered states of mind. Most famously, in The Fan (1996), a baseball fanatic pursues his hero to the point of madness. Robert DeNiro and Wesley Snipes starred in the film version directed by Tony Scott. 

In Oblivion (2005), Abrahams explores the altered state of mind that follows removal of a brain tumor. The chief symptom is loss of memory for the three days leading up to the surgery.

The loss is critical because the hero is a private investigator tracking a missing-person, and he spent those three days getting close to cracking the case. Following his surgery, he must re-investigate the case, but all the people he talks to now know more about what he's up to than he does. This puts him in increasingly dangerous situations.

To complicate matters, years earlier, one of his cases was the subject of a based-on-a-true-story Hollywood film, and it's not always clear whether people are remembering him or the character based on him in the movie. 

Alteration of the hero's state of mind makes this one of the more ambitious P. I. novels I've read.