La Casa del Libro Museum Library (LCDL) is a museum library where the written word and the book as an object of art are worshipped. It houses the largest and most important rare books collection (rare because of rarity, age, edition, historical value, craftsmanship, provenance, or subject matter) in Puerto Rico and the Caribbean, including "incunabula", a Latin word for ‘cradle’ since they were printed between 1450 and 1501, during the early days of modern printing.

La Casa del Libro, a non-for-profit organization, offers a dynamic program of exhibitions, educational activities, book arts workshops, and book readings through which it seeks to bring the humanities closer to the community. This way, LCDL contributes to the reflection and understanding of the world heritage by all sectors: students, teachers, neighbors, travelers, the elderly, and disadvantaged groups.

It was founded by Elmer Adler (1884-1962), a printer, publisher, editor, book designer, author, and an avid collector of books and fine prints. In 1954, Adler visited Puerto Rico after retiring from Princeton University, where he established a department of graphic arts and served as a lecturer and library curator. He stayed to lead and organize LCDL and its formidable collection, which he built up book by book through donations.