Picked up in a trolley-load of books from Molyneux.

Pre-numbered label used for registration.

Oh heck, another one I read *ages* ago and yet somehow didn't journal.
I'd read K-Pax before so skipped that volume. That left the matter of prot's existence ambiguous, although siding more with the psychologist's view that he was just a personality construct.
The next two books make things much less clear for Dr Brewer, providing him with proveable physiological differences between prot and his human side, Robert, despite the increasing amount of evidence for the alien's existence being caused by trauma in Robert's past.
Because it's all written in the style of reports, Dr Brewer's narration is often undeservedly smug and blinkered to the admittedly weird wonder of the situation he finds himself in, but that often provides 'well duh!' moments for the reader as the scientist is forced to remember the humanity of his subjects - and the failings of his field.
And no, you're still not sure by the end of it all whether there is an alien borrowing a human body, or it's all a dleusion.