This book, despite the ostentatious title which Tolkien himself might've disavowed (he might humbly have thought that the Bible and other works, not his own books, were the true "books to rule them all"), is well worth reading.
It covers many aspects of philosophy and thought, including Plato, Nietzche, existentialism, Eastern religion, etc., which do not always receive the discussion vis-a-vis Tolkien that they deserve.
One of the best essays is Alison Milbank's "'My Precious': Tolkien's Fetishized Ring", an analysis which resembles Brenda Partridge's (in)famous 1983-or-so essay "No Sex, Please, We're Hobbits: The Construction of Female Sexuality in The Lord of the Rings", in its commentary on Shelob's scary voracity. Milbank also mentions Karl Marx's "commodity fetishism" as a factor in Tolkien's work (and the Ruling Ring is certainly one hot commodity in Middle-earth, even before Frodo "gives Gollum the finger" on Mount Doom and the action heats up a bit)...though Milbank notes that Tolkien probably had no "People's Republic of the Shire" in mind when writing Lord of the Rings!!
Another standout essay is "Happy Endings and Religious Hope: The Lord of the Rings as an Epic Fairy Tale" by John J. Davenport. Of all the essays, it perhaps draws most deeply on a variety of Tolkien's works, including the Silmarillion and Tolkien's influential essay "On Fairy-Stories". Davenport, whose essay is the last in the book (and, significantly, at the end of the "Ends and Endings" group of essays), poses the hope that "Day will come again" ("Aure entuluva" in the Elvish spoken at a desperate battle in the Silmarillion) not only in Middle-earth but also on our own earth, at least from Tolkien's Christian point of view which hopes for eventual reward for those who strive for right throughout their lives.
Davenport ably invokes the Beowulf epic, the tales of King Arthur, and the Tolkien-favorite medieval story of "Sir Gawain and the Green Knight" in showing how Tolkien's goal of finding "joy, poignant as grief" is forwarded through The Lord of the Rings' combination of epic narrative with "eucatastrophe", Tolkien's brilliant term meaning more-or-less "a catastrophe of good" or "a surprise turn for the better, such as found in fairy tales". And indeed, as Davenport notes, the various "eucatastrophes" in Tolkien's trilogy do leave one with a taste of hope for something better in our futures, dark as the interim may be.
Back to the book as a whole: although the still photo of the resurrected Gandalf from the Two Towers film gracing the cover looks a little cheesy (though still impressive), the light-from-above in the picture does remind us that there is something gleaming or "eternal" caught in the mesh of Tolkien's work, not mere idle fantasy. Though lacking the coherence and focus that a book-length piece would have, as opposed to the various scattered and short essays in "One Book to Rule Them All"--and I was sorely tempted to give only 4 stars for this book, because of this scatteredness--, "One Book" does a fine job of reminding us of the genius of Tolkien not only for entertaining narrative but also for offering serious thought about the meaning of life, and "One Book" does so all the better by its drawing on his fellow geniuses throughout the millenia to illustrate or complement his points.
Two thumbs up (and any ring-fingers left on one's hand).