Herland by Charlotte Perkins Gilman
Three young men discover a hidden land peopled entirely by women (they reproduce by parthenogenesis). These women have built a sort of progressive-collectivist utopia, much to the chagrin of at least one of the men who cannot believe there can be a civilization without men. Although Gilman spends a little too much time in exposition, there are numerous telling observations on the limitations imposed on both men and women by conventional gender roles, and the absurdities and abuses allowed by a patriarchal society. Two of the men and one woman are setting out to return to the outside world in the end, and I feel Gilman could well have extended her narrative to show the woman from Herland's culture shock out in our world, instead of abruptly ending it at this point. In short, a witty, thought-provoking and interesting book, but one that falls short of its full potential.
ETA: Actually, there is a sequel, With Her in Ourland , which forces me to increase my rating of this book.