The Legal Theory Bookworm recommends In Defense of Plural Marriage by Ronald C. Den Otter. Here is a description:
With over half of Americans now in favor of marriage equality, it is clear that societal norms of marriage are being quickly redefined. The growing belief that the state may not discriminate against gays and lesbians calls into question whether the state may limit other types of marital unions, including plural marriage. While much has been written about same-sex marriage, as of yet there has been no book-length legal treatment of unions among three or more individuals. The first major study on plural marriage and the law, In Defense of Plural Marriage begins to fill this lacuna in the scholarly literature. Ronald C. Den Otter shows how the constitutional arguments that support the option of plural marriage are stronger than those against. Ultimately, he proposes a new semi-contractual marital model that would provide legal recognition for a wide range of intimate relationships.