Overall, this is a well-written book, which draws on both traditional exegesis and modern literary scholarship. In fact, one of Ziegler's points is that these two approaches are often complementary, and come to the same conclusions based on noticing the same initial unusual text features, just using different language to connect the source and the conclusion. (I think that Rabbi Nathaniel Helfgot, in his book "Mikra and Meaning", also published by Koren/Maggid, makes the same point more concisely and effectively, but it's a good point to reiterate.)
There are three main themes in this book. First is the narrative of Ruth serving as a "tikkun", a repair, for the descent of the nation throughout the book of Judges. Second is a focus on identity versus anonymity, and the importance of when people are known by name and when they are identified merely as a type. Third is comparing passages in Ruth with parallel passages elsewhere in Tanach, both on a narrative level and in terms of word choice.
Ziegler is effective in all three of these areas. She expresses her ideas crisply. She clearly has deep familiarity with a broad base of sources on which to draw, and a keen sense of when an unexpected word choice or subtle parallel to another biblical text can unlock an entirely new way of understanding a deceptively straighforward text.
There are a few weaknesses in this book.
While it functions as a commentary on the Book of Ruth, and each chapter (really, each essay) begins by citing the relevant verses (in Ziegler's own excellent translation), it is not clear to the reader whether these excerpts combine to form the complete text of Ruth. (Also, the text of Ruth is short enough that each essay's primary source text could and should have been included in the Hebrew as well.) Because Ziegler sometimes has several essays on the same (or overlapping) passages, exploring them from different angles, we find ourselves sometimes jumping ahead, sometimes doubling back. I found that confusing, especially in the section on Chapter Two.
Relatedly, because the book started as a series of essays in the Yeshivat Har Etzion online beit midrash, it suffers a little bit from redundency -- again, mostly in the section on Chapter Two. I wish there had been some more intense editing here, both in terms of combining related essays and, frankly, distilling some of the supporting material. When each essay had to stand on its own as a separate email thread, the reiteration of key arguments in full was necessary; in a book, it becomes dulling. At nearly 500 pages, this book outweighs the source material in the Book of Ruth by a factor of fifty; at some point, the sheer mass of it overwhelms Ziegler's otherwise skillful writing.
Ziegler has some deeply insightful things to say, and I will never be able to read "Ruth" as a simple narrative in isolation again. There are several passages in the book which I have bookmarked as sources for potential divrei Torah when Shavuot comes around. It is worth struggling through the slowness in the middle to learn from a scholar with her finger on widely disparate sources and an ear for the subtleties of language that make the Book of Ruth much richer than we realize when we listen to it only once a year.