Of American-English origin:
– the noun bibliotherapy designates the use of books for therapeutic purposes, especially in the treatment of mental health conditions;
– the noun bibliotherapist designates a practitioner of bibliotherapy.

The noun bibliotherapy occurs, for example, in There was a 17^th-century version of FOMO, by Stephen Asma, published in the Boston Sunday Globe (Boston, Massachusetts, USA) of Sunday 6^th August 2023 [Vol. 304, No. 37, page K8, column 2]—the following is about The Anatomy of Melancholy (1621), by the English author Robert Burton (1577-1640):

Burton [argued] that loneliness was a major cause of depression. Friends, he argued, had a duty to step in and uplift the struggling soul, with optimistic company and care. Music and mirth are listed as tools friends can use to help relieve one another. And when friends can’t be found, then reading and writing can be very therapeutic (psychologists now call this bibliotherapy and scriptotherapy).

The earliest occurrences of bibliotherapy that I have found are as follows, in chronological order—the first three texts indicate that this noun was coined by the U.S. essayist and Unitarian minister Samuel McChord Crothers (1857-1927):

1-: From an account of the fourteenth annual meeting of the Keystone State Library Association, held at Galen Hall, Wernersville, from Thursday 15^th to Saturday 17^th October 1914, published in Pennsylvania Library Notes (Harrisburg, Pennsylvania: Pennsylvania Free Library Commission) of October 1914 [Vol. 7, No. 4, page 108]:

Dr. Samuel McChord Crothers […] spoke on “The Therapeutic Value of Books,” saying, in substance:
[…]
The profession of a librarian means not simply the preserving of books or making selections for other people, but uplifting the intellectual life of a community through the medium of books. The librarian’s science might be termed bibliotherapy. He should treat people who come to the library as patients who come with various kinds of maladies, most suffering from mal-nutrition. Librarians know how to catalogue books in many ways—all of them good, although in this connection I am reminded of the old saying: “All deacons are good, though there is a choice of deacons.” Librarians should put the same mental effort into the consideration of their borrowers’ desires as they do upon the catalogue. Too often is the ultimate consumer of the book left out of account, when in reality every book does something to the reader—if only to put him to sleep. There is positive therapeutic power in a book, and it is interesting to compare notes on the effect of the book while reading and after. Treat a borrower as a physician treats his case, and watch the reactions of different books.

2-: From the Daily Gazette-Times (Corvallis, Oregon, USA) of Friday 18^th February 1916 [Vol. 7, No. 253, page 4, column 4]—O.A.C. stands for Oregon Agricultural College:

Dr. Crothers Talks To O.A.C. Students

“Books are a stimulant, a saporific [sic], a sedative, a corrective, an irritant or a counter-irritant. . . . . Young people form the poison squad for the new books, if they survive, the older people get the contagion.”
Such was one of the laws of “literary therapeutics,” as stated at Convocation Wednesday, by Dr. Samuel McChord Crothers, research lecturer, essayist, philosopher, in his exposition of “A Literary Clinic.”
The central theme of Dr. Trother’s [sic] address concerned the activities of “his good friend, Baxter,” [sic] who conceived the idea of curing mental ailments by a correct choice of books to read, by “Bibliotherapy;” as physical ailments are cured by, “Psychotherapy.” [sic]

3-: From A Literary Clinic, by Samuel McChord Crothers, published in The Atlantic Monthly: A Magazine of Literature, Science, Art, and Politics (Boston and New York: The Atlantic Monthly Company) of September 1916 [Vol. 118, No. 3, page 295, column 1]—in this essay, Samuel McChord Crothers poses himself as a narrator and carries out an amusing interview with a fictional practitioner of bibliotherapy, Dr. Bagster, who says the following to him:

‘Bibliotherapy is such a new science that it is no wonder that there are many erroneous opinions as to the actual effect which any particular book may have.’

4-: From the column Bits for Breakfast, by George Douglas, published in the San Francisco Chronicle (San Francisco, California, USA) of Tuesday 22^nd October 1918 [Vol. 113, No. 99, page 16, column 5]:

“Bibliotherapy” is the immortal coinage of Christopher Morley in a sequel to “Parnassus on Wheels.” Blessed word! What a lot of ailments can be cured by a course of good reading!

5-: From The Haunted Bookshop (Garden City, New York: Doubleday, Page & Company, 1919), a novel by the U.S. author and journalist Christopher Morley (1890-1957):

The young man had heard of none of these books prescribed by the practitioner of bibliotherapy.

The earliest occurrence of the noun bibliotherapist that I have found is from an account of the Hospital Libraries Round Table meeting, held in New Orleans, Louisiana, on Tuesday 26^th April 1932, from Proceedings of the Fifty-fourth Annual Conference, published in the Bulletin of the American Library Association (Chicago, Illinois: American Library Association) of August 1932 [Vol. 26, No. 8, page 565, column 1]:

Florence Sytz, […] a psychiatric social worker[…] had consented to speak to the hospital library group. Her address follows:
Adapting Bibliotherapy to the Patient’s Needs in the Changing Social Order
In many ways this is a queer subject for me to be talking about, for I am neither a bibliotherapist nor an “expert” on the “changing social order.”