Answer by taking a quote from the following article:

Henry Havelock Ellis, known as Havelock Ellis (2 February 1859 - 8 July 1939), was an English physician, writer, progressive intellectual and social reformer who studied human sexuality. He co-authored the first medical textbook in English on homosexuality in 1897, and also published works on a variety of sexual practices and inclinations, as well as on transgender psychology. He is credited with introducing the notions of narcissism and autoeroticism, later adopted by psychoanalysis. Ellis was among the pioneering investigators of psychedelic drugs and the author of one of the first written reports to the public about an experience with mescaline, which he conducted on himself in 1896.

Dr. Havelock Ellis' 1933 book, Psychology of Sex, is one of the many manifestations of his interest in human sexuality. In this book, he goes into vivid detail of how children can experience sexuality differently in terms of time and intensity. He mentions that it was previously believed that, in childhood, humans had no sex impulse at all. "If it is possible to maintain that the sex impulse has no normal existence in early life, then every manifestation of it at that period must be 'perverse,'" he adds. He continues by stating that, even in the early development and lower function levels of the genitalia, there is a wide range of variation in terms of sexual stimulation. He claims that the ability of some infants producing genital reactions, seen as "reflex signs of irritation" are typically not vividly remembered. Since the details of these manifestations are not remembered, there is no possible way to determine them as pleasurable. However, Ellis claims that many people of both sexes can recall having agreeable sensations with the genitalia as a child. "They are not (as is sometimes imagined) repressed." They are, however, not usually mentioned to adults. Ellis argues that they typically stand out and are remembered for the sole contrast of the intense encounter to any other ordinary experience.  Ellis claims that sexual self-excitement is known to happen at an early age. He references authors like Marc, Fonssagrives, and Perez in France who published their findings in the nineteenth century. These early ages are not strictly limited to ages close to puberty as can be seen in their findings. These authors provide cases for children of both sexes who have masturbated from the age of three or four. Ellis references Robie's findings that boys' first sex feelings appear between the ages of five and fourteen. For girls, this age ranges from eight to nineteen. For both sexes, these first sexual experiences arise more frequently during the later years as opposed to the earlier years. Ellis then references Hamilton's studies that found twenty percent of males and fourteen percent of females have pleasurable experiences with their sex organs before the age of six. This is only supplemented by Ellis' reference to Katharine Davis' studies, which found that twenty to twenty-nine percent of boys and forty-nine to fifty-one percent were masturbating by the age of eleven. However, in the next three years after, boys' percentages exceeded that of girls.  Dr. Ellis also contributed to the idea of varying levels of sexual excitation. He asserts it is a mistake to assume all children experience are able to experience genital arousal or pleasurable erotic sensations. He proposes cases where an innocent child is led to believe that stimulation of the genitalia will result in a pleasurable erection. Some of these children may fail and not be able to experience this either pleasure or an erection until puberty. Ellis concludes, then, that children are capable of a "wide range of genital and sexual aptitude." Ellis even considers ancestry as contributions to different sexual excitation levels, stating that children of "more unsound heredity" and/or hypersexual parents are "more precociously excitable."

What was the name of the book?
Psychology of Sex,