Hermann Ebbinghaus (1850-1909) was a German psychologist who pioneered the experimental study of memory, and is known for his discovery of the forgetting curve and the spacing effect. He was also the first person to describe the learning curve. In 1885, he published his groundbreaking Über das Gedächtnis ("On Memory", later translated to English as "Memory: A Contribution to Experimental Psychology") in which he described experiments he conducted on himself to describe the processes of learning and forgetting. Ebbinghaus made several findings that are still relevant and supported to this day. First, arguably his most famous finding, the forgetting curve. The forgetting curve describes the exponential curve that illustrates how fast we tend to forget the information we had learned. The sharpest decline is in the first twenty minutes, then in the first hour, and then the curve evens off after about one day.
Disclosure: I am a memory researcher and book is of particular historical interest for me. I thought the 1913 translation was great; it's easy to see why it has stood the test of time, with multiple reprints in the 21st century. This guy was unbelievably prescient and he would be pleased to see the course that modern science has taken because it is so like his own conception of it. A true pioneer of social science research, quantitative psychology, and experimental design, Ebbinghaus sounded eerily modern throughout. His ideas and discoveries abound in such a way that you are left wondering what memory researchers have been up to for the past 150 years! In this book, he either postulates or hypothesizes about lots of big ideas, many of which still provide reliable fodder for research programs today: the testing effect, the spacing effect, connectionism, the learning (and forgetting) curves, mathematical modeling of psychological phenomena, the serial position effect, observer-expectancy effects, blinded experimental designs... Not only that, but he treats several of them as if they were old ideas or so much common sense. The ending was abrupt, but hey: it's a translation of a scientific monograph from the 1880s after all. Still, it was eminently readable, particularly for those already inured to reading the results sections of scientific journals (the received tradition of which Ebbinghaus appears to have influenced in the way he organized and reported his results, evaluated his hypotheses, and discussed the implications of his findings). His rigorous self-experimentation is an important precursor of the quantified-self movement, with people like gwern now bearing the standard and carrying on Ebbinghaus' lofty legacy (eg., https://www.gwern.net/#qs).
The level of dedication and work put in by Ebbinghaus is incredibly impressive. However, the book was not an easy read and would likely be more comprehensible if there was a more modern translation. Ignoring readability though, the prose sounded nice.