This work presents the first detailed study of the construction and materials of the Stockholm Public Library. As the building undergoes a rare period of maintenance and renovation, the floors and walls of the library are examined from three perspectives. First, using available but limited archival documents and plans; second, with non-destructive ground-penetrating radar measurements; and finally, through on-site surveys during local interventions for the maintenance and renovation process. The ensuing results emphasize the complementary nature of this combined research approach in recovering lost or forgotten construction details and further reveal several important findings. In the case of the unique wall finishing of the library’s rotunda, multiple layers of lime mortar, each varying in thickness and coarseness, were used to build up and craft the relief-like interior wall surface. With the use of in-situ aerated concrete and prefabricated Solomite panels in the library’s 1931–32 floor construction, a material connection between Asplund and the broader modern movement in architecture is further highlighted. At first glance, these construction-related findings seem to reinforce the common architectural narrative of the library as a transitional project between neoclassicism and modernism. At the same time, however, the library’s separate periods of construction of 1925–28 and 1931–32 and their distinct materials can be seen as a continuity of construction culture, with the innovative use of local raw materials related to the Swedish landscape.