Iris: A Novelette by J.A. Glass

https://www.goodreads.com/book/show/22715020-iris

IRIS documents the encounter between an aging photographer and a divorced therapist. 
The fall from sanity is a steep one as Dr. Mark Mason tries to piece together the mind of his newest patient, Iris Davis. Dr. Mason must fight to distinguish memories from the twisted labyrinth of her decaying mind before their time runs out.