More Gender Trouble (1990) with Jennifer Hansen. We get into the metaphysics of substance (are gender and sex attributes that a person has, or is there a better way to describe the situation?), performatives, and what Butler sees as the available mechanisms for changing gender norms.
We compare the different views of femininity within male-defined conceptual space according to Beauvoir (for whom the female is defined as the lack of male attributes) and Luce Irigaray (for whom even this "lack" is another aspect of maleness, with the feminine literally unspeakable, i.e., there's no room for it at all in our current scheme). Another go-to figure for Butler is Monique Wittig, whose analysis labeled compulsory heterosexuality (in the service of propagation of the species) as the primary force driving the creation of binary gender concepts. She also talks about Foucault, on three counts: First, in his History of Sexuality, he gives a picture of humanity as natively pansexual, but then "The Law" comes and creates a structure by which both (approved) heterosexuality and (forbidden) homosexuality are articulated. Second, he adapted Nietzsche's idea of philosophy as genealogy to its modern form, which Butler says is her goal in this book even though her story of the origins of gender lacks the concrete historical detail that fills Foucault's books. Finally, Butler talks a lot about Foucault's introduction to the memoir of Herculine Barbin, a "Nineteenth Century French Hermaphrodite," which demonstrated for Foucault a gender identity less wholly determined by—in fact, in explicit conflict with—social norms than is the case for most people. Butler uses all of these other figures as dialectical steps in her critique: They all have something right and worth articulating in them but are then in turn critiqued for not seeing all the logical conclusions of their views.
Buy the book. We also found these earlier articles by Butler very helpful: "Performative Acts and Gender Constitution" (1988) and "Sex and Gender in Simone de Beauvoir's Second Sex" (1986). For a more confident and detailed argument for the thesis that biological sex is socially constructed (in the current book it's more raised as a question), try her subsequent book, Bodies That Matter (1993). For a more direct, recent treatment of trans* issues by Butler, see this 2015 talk.