Selman Waksman's HUMUS: Origin, Chemical Composition, and Importance in Nature (1936)
Selman Waksman, a microbiologist who won the Nobel Prize in 1952 for the discovery of streptomycin, wrote this thorough and well-researched book on humus in 1936. It is available as a 21.6 mb pdf (text-searchable) download here.
Though the basics of carbon cycling, including microbial decomposition of soil organic matter and humus, had been established before, Waksman's work clarified the important roles of microbes in the formation and decomposition of soil organic matter. The book is a major milestone in the ongoing discovery of the carbon cycle.
"Humus plays a leading part in the storage of energy of solar origin on the surface of the earth."