The Ethical Slut is one of the books most frequently recommended to anybody interested in polyamory. That is partly because, written over 20 years ago, it established its reputation at a time when there were few alternative resources. And it is partly because, even now, the alternatives are less than perfect. More than Two, another prominent book on poly, lost its lustre when it emergeed that one author had abused multiple women, including his co-author
When I mentioned to friends what I was reading, I heard much more criticism than praise. Similarly, once our book group held its discussion, we ended up with objections on all sides.
One complaint is the way that The Ethical Slut discusses other minority sexual groups. Despite being written by two bisexual/lesbian women, it feels very targeted towards a heterosexual couple considering opening up their relationship. When it discussed gay and lesbian communities, it is in a way that over-generalizes from the groups the authors have encountered.
A criticism I care less about is historical accuracy. The Ethical Slut includes a whirlwind tour of groups who have in the past practiced non-monogamy. Treat it as a historical treatise and you will be infuriated. I read it more as a reminder that the mid-20th-century American nuclear family is not the single model by which all human society has arranged itself.
Mostly, I feel the people in my circle who dislike The Ethical Slut want it to be something it is not. Somebody already deeply embedded in LGBT, kinky, and poly groups is unlikely to find the book frustratingly basic. But if you come to the book from the straight, monogamous mainstream, this is perhaps the introduction which you need.