Fantasy and science fiction aren't everyone's cup of tea reading-wise, but those genres do tend to be more poly-friendly than most. In Robert Heinlein's science fiction novel Friday, the title heroine is involved a in a group marriage at the beginning of the book. In the Sharing Knife series by Lois McMaster Bujold, the hero is friends with a woman and two men who are all bound to one another by the same technique their people use to bind monogamous husbands and wives, and the children of the triad are considered the children of both men because no one, including the triad themselves, knows or cares which man *biologically* fathered any of the children.
So yeah... if your wife likes fantasy and science fiction, that might be a resource. Some people don't enjoy those genres; other people, like me, despise reading nonfiction and so can learn a lot from fiction that depicts such things, as long as one is aware that fiction isn't always accurate.
(I wrote, years ago, a contemporary novel in which a woman who's living with her fiance reconnects with an old lover, and the three of them navigate transitioning into a V with the woman as a hinge... but this isn't a place for advertising, and the book's been out of print for over a year now because I got fed up with the publisher. It got good reviews, though, including several people who said it was one of the most accurate depictions of polyamory they'd seen in fiction...And that was before I started doing poly myself.)