Tolkien scholarship, on the whole, tends to be quite weak. There's a lot of reasons for that, but this isn't really the place to go into them. However, I will say that this collection of essays is no exception to the rule. "Tolkien: A Celebration" consists of 15 essays by different authors, edited by Joseph Pearce (author of another mediocre book on Tolkien). For the most part, the essays are non-scholarly-- they are more like short, off-the-cuff, reflections than serious scholarly analysis.
Most of the essays deal in some way with the "religious" aspects of Tolkien's fiction-- and most of those approach it from a specifically Roman Catholic persepctive. This is a legitimate subject to write about, of course, but it's been done to death before (and better!) by Carpenter, by Kocher, by Kilby, by Flieger, and by a host of other critics. These essays really don't add anything new to the body of Tolkien scholarship-- no new ideas, no new interpretations, no new evidence.
The same is true for most of the non-religious-themed essays as well. Patrick Curry's "Modernity in Middle-Earth", for example, is basically a six-page summary of his own book on the subject, while Elwin Fairburn's "A Mythology for England" is essentially a recap of points that have been made again and again and again by previous scholars (especially Carpenter, and even more Jane Chance who wrote a whole book called "Tolkien's Art: A Mythology for England").
In truth, the only two items of genuine interest here are the "personal reminscences" by George Sayer and Walter Hooper, who talk abou their experiences meeting Tolkien, working with him, etc., They're not rigorous scholarship,
nor do they present a radically different picture of the man than Grotta-Kurska's and Carpenter's biographies draw, but they do offer up a few worthy anecdotes. Still, they're hardly essential reading for either the Tolkien scholar or fan.
This isn't, by far, the wost book on Tolkien ever published, but it's not one of the better ones-- and it really doesn't have anything new to add to the critical legacy of Tolkien scholarship.