Über das Gedachtnis: Untersuchungen zur experimentellen Psychologie.

Ebbinghaus conducted a series of experiments on memory and forgetting using himself as subject. Out of this came concepts such as the learning curve and the forgetting curve, and the serial position effect. Digital facsimile from Google Books at this link. Translated into English by Henry A. Ruger & Clara E. Bussenius as Memory: A contribution to experimental psychology (1913), of which the digital text is available from Classics in the History of Psychology at this link.