Although the term they introduced came to be completely associated with sexual relationships outside of marriage, the book is largely an argument that marriage can be most effective if each partner has a clear and continuing life that does not depend entirely on the partnership. The book mainly argues for partners, especially the women in a partnership, to continue pursuing their interests, hobbies, intellectual passions, and careers, with the same intensity they would if not married. The book's suggestion is that such full lives for each partner brings continually growing and interesting persons to the evolving marriage. Early in our life, Bev and I found the ideas in this book very useful, although they now seem a little like common wisdom in a world where feminism and self-actualization are more common priorities than when the O'Neills wrote this.
I understand that the O'Neills were aghast that their term came to be synonymous with non-monogamy. Anthropologists are rarely revolutionaries. The NYT says that "Three of the book's 287 pages explore, ever so tentatively, the elastic properties of marital fidelity. Forever after, those pages were all anyone seemed to remember about "Open Marriage."
I bought this to read both because it's a classic in the field of non-monogamy and because it's a interesting historical document, having been published in 1972. I read the first chapter "Why Save Marriage At All?" and it's promising: "Only by writing their own open contract can couples achieve the flexibility they need to grow." The bookseller who sold it to me immediately offered both that "this book made quite a splash in its time" and "I knew someone who tried it - it ruined their marriage" - of course. LOL