I believe that many of the reviews for this book have really missed the point. If you view this book outside its historical context, there is no way by which you will understand it. Indeed, claims towards its misogyny and anti-Semitism must be put into context if we are to understand why it is that this young, sexually-confused, and Jewish man decided to write this seemingly bizarre piece at the turn of the 20th century. In particular, those who have read Michel Foucault's works on sexuality and psychoanalysis will find this work incredibly interesting.
While the work begins with a very modern conceptualization of sex, it reverts back to an extremely misogynistic view later in the book. By this I mean, whereas Weininger points to the fact that terms such as 'male' and 'female' are extremely imprecise, and that every individual fits in between this dichotomy, he later reverts to the opposite position, invoking terms that he himself diagnosed as being absolutes: 'Man', 'Woman', 'Male', and 'Female'. Thus, I believe that the interesting question with this book is the following: why does Weininger both point to a theory of bisexuality early in his work, and stick to an absolute model of sexuality later in the work?
This is very interesting from the point of the history of psychology and sexuality because as several scholars have pointed out, Freud falls into the same trap within his "Three Essays on the Theory of Sexuality" in attempting to define masculine and feminine sexual instincts.
Working off the works of Michel Foucault and Arnold Davidson, I believe that these contradictory theories of sexuality point to a fundamental change that was occuring in both the field of psychology and in theories of sexuality during the early 20th century. Previous models of sexuality and sex that relied upon anatomical distinctions began to be turned into functional representations. Thus, in this conceptual framework, sex became an instinct innate to the body, which identified or characterized a type of subject.
Indeed, Weininger argued that the foundation of psychology must move towards interpretation and away from mere sensations or biological specificities, thus placing into question the whole of one's character or personality. Whereas psychiatrists such as M.P. Legrain and Sigmund Freud located individual personalities to a certain extent in the sexual instinct, Weininger goes further to seat this within sex itself.
Towards the end of the book, Weininger goes beyond merely analyzing individual genders, for he applies these archetypal notions of `man' and `woman', or `masculine' and `feminine', to the idea of civilizations and races in later chapters of the book, paying particular attention to Judaism and the Jewish people. In order for this to have made conceptual sense, the concept of sex must have, at the least, broken with a specifically biological model of the sexes. Notions of `male' and `female' had to have become detached from anatomy and, instead, applicable to and determinate of the character of individual societies and classes, to name only a few examples.
In other words, in Weininger's book, one concretely observes the manner in which sex became problematized during the late 19th and early 20th century. Weininger disassociates sex from its biological manifestation and introduces an interprative approach that was to become the hallmark of psychoanlysis. Consequently, I do suggest you read this if you are interested in the history of psychoanalysis and theories of sexuality.