Los Renglones Torcidos de Dios (God's Crooked Lines) by Torcuato Luca de Tena.

 A female private detective who goes into a mental institution to solve a murder case. For that she has to pretend to be paranoid. But, is she really pretending or was she sectioned because she is ill? This novel really surprised me. It is well written, intriguing, addictive. The main character is superb. She makes reading the whole novel worth it. She is intelligent, charming and paranoid? Her situation/condition plays a much bigger role than the murder case and it is what makes you want to keep reading. But it doesn't end there. The setup, among psychiatrists and the mentally ill, is really good. To write this book the author visited several mental hospitals and then got himself admitted in one for 18 days. It is from these experiences that he got inspiration for the many mentally ill patients and psychiatrists in the novel. It's a pity that there isn't an English translation, but there is a Netflix adaptation which might be worth watching.