The second edition opens with a great little introduction by Professor M. Nussbaum, who helps tie this work contextually to similar works in the study of sexuality (Greek, Roman and modern) written in the last twenty years. She demonstrates the reasons for reading this work in particular.
Williams is as thorough-going as I could ask for. His work not only provides an understanding of the dynamics of Roman non-heterosexual practices, it also illuminates the basic framework of theories of desire and love, as well as the apparent legal structures surrounding and checking the sexual acts described. The work is a successful partner to Dover's Greek Homosexuality.
William's work also introduces the reader to much of the Roman sexual vocabulary, and includes scores of passages from relevant verse. I'm tackling Catullus just now, and, for better or worse, I understand precisely what it means to tell a Roman, "irrumabo"--both literally and in light of its social implications.
The book also attempts an analysis of male gender. Since this isn't totally coupled with a male sexual practices (in my opinion), Williams is smart not to press the point to dearly. It isn't his central thesis, yet he develops it so that he can correctly tie it to the thesis he's defending.