The De Jong collection includes over a thousand underground and illegal publications that appeared in the Netherlands between 1940 and 1945. During the postwar years, Dirk de Jong collected them and described them in his standard work Het vrije boek in onvrije tijd (The free book in unfree times), which was published in 1958. It is an exhaustive survey of underground publications in the Netherlands during the Nazi occupation.

Most of the publications are belletristic, but there are also some political or purely informative publications in the collection. For the most part, the books and brochures were typed in all secrecy at home and then stenciled or printed on private presses. Nevertheless, some of them are genuine works of art, printed on fine paper and illustrated, and featuring engaging or personal texts from Dutch and foreign authors.

The printing run was limited and the copies went from hand to hand. Many copies got lost during or after the war. This makes the De Jong collection quite unique. It is administered by the Library of the University of Leiden and is nowadays still supplemented with illegal and underground publications that had not come to De Jong's attention.