The concept is not new - Psychology Today reports that Egyptian King Rames II had a special chamber for books, with the words "House of Healing for the Soul" over the door. Evidently Sigmund Freud also used literature in psychoanalysis, and other doctors have "prescribed" books for their patients. Apparently now it's a thing, and some psychologists train in it.
To me, bibliotherapy seems like another one of those intuitive and obvious things that has now been turned into an industry. It reminds me of Forest Bathing, an entire science devoted to the shocking notion that spending time outside is good for you.
To be clear, bibliotherapy does not refer to self-help books. We're talking about novels which portray the human condition.
As soon as I learned to read, I stayed up late at night, under the covers with a flashlight, spending hours on adventures with Doctor Doolittle, devouring stories about families and orphans - The Little Princess was big for me - and on and on. As my reading abilities and sophistication increased, so did the depth of my book choices. Books have always served as both an escape and a way of understanding the world.
It makes sense that being a good reader helps develop empathy in a person, but apparently it can help with depression, anxiety and other human conditions. Who knew? Most readers.