The Silent Patient by way of Stephen King: Parker, a young, overconfident psychiatrist new to his job at a mental asylum miscalculates catastrophically when he undertakes curing a mysterious and profoundly dangerous patient.
In a series of online posts, Parker H., a young psychiatrist, chronicles the harrowing account of his time working at a dreary mental hospital in New England. Through this internet message board, Parker hopes to communicate with the world his effort to cure one bewildering patient.We learn, as Parker did on his first day at the hospital, of the facility’s most difficult, profoundly dangerous case—a forty-year-old man who was originally admitted to the hospital at age six. This patient has no known diagnosis. His symptoms seem to evolve over time. Every person who has attempted to treat him has been driven to madness or suicide.
Desperate and fearful, the hospital’s directors keep him strictly confined and allow minimal contact with staff for their own safety, convinced that releasing him would unleash catastrophe upon the outside world. Parker, brilliant and overconfident, takes it upon himself to discover what ails this patient and finally cure him. But from his first encounter with the mysterious patient, things spiral out of control and, facing a possibility beyond his wildest imaginings, Parker is forced to question everything he thought he knew.
Fans of Sarah Pinborough’s Behind Her Eyes and Paul Tremblay’s The Cabin at the End of the World will be riveted by Jasper DeWitt’s astonishing debut.

REVIEW AS A READER:
I researched "best psychological thrillers" on Bard.google.com and this book, The Patient, came up as number one. I read through the Amazon description, but I chose it based on Bard's recommendation.
The psychological mystery and drama started from the first page. The mystery was subtle at first, but it was definitely there. I was hooked and wanted to know more from the first page. The story unfolded slowly but it was never boring.
Reading the book was like driving on long winding roads, not knowing the ultimate destination, and also not being sure what lay right around the bend. The drama moved along without stop. The story never sagged or drove off onto some boring side road. The mystery and suspense were constant.
There was an intimacy in the story as some of it was written in first person. Other parts were written in third person where the story took on more of a bird's eye view and a bit of narration. It all worked and added to the mystique of the book.
I did feel that at the end of the book, the ending veered off into a bit of unexpected sci-fi or fantasy I was not expecting, but it was still an enjoyable book from start to finish.