Being an symposium on the topic suggested by the title. Integrity demands that I mention here a slight but friendly on-line acquaintance with Ms Higgins.
There are two ways to review a book of this sort -- a lengthy discussion of each article in the book; or to simply make some remarks about it as a whole. As I have neither the energy nor the background to do the first, I'll try to make the best I can of the second.
To begin with, this is a seriously academic volume (which is where my background lacks; I was a techie in school and, to the extent I know anything about literature and criticism, it's autodidactic). There are, at least, two possible pitfalls for this for a casual reader.
The most obvious is that academic papers are frequently written at a forty-seventh grade reading level, with a burgeoning of hypersesquepedalianisms and tortured, involved, and difficult to follow, if only because it takes side trips before getting to its point, syntax.
These essays, as a whole, do not fall into that particular pit. Though there are occasional terms an average reader won't know, they're mostly made pretty clear, if not by definition, then by their usage. And the sentences are clear.
The other pitfall is a kind of vanity of thought. More than one literary essay I've read comes up with an interesting theory of the meaning, symbolism, or what-have-you, the writer(s) of the text(s) under examination used in creating them.
Now, I personally hold to the position that, if, when reading with respect and integrity, you find something in the text, it is perfectly legitimately there, if only for you. Once a text is published, it no longer belongs to the writer, but to the world, and, while the writer certainly has the right to declare what she meant when writing, she is no longer the sole authority on what it actually says. Another writer (reviewer, essayist, critic, whatever) has every right to say that, in interpreting the text before them, they see aspects X, Y, and Z, which nobody has noticed before.
What they do _not_ have, I think, is the right to say that the writer _meant_ for X, Y, and Z to be there. This is a dangerous position to take, because it is essentially an attempt to read the writer's mind. This is obviously a pitfall for the essayist, but why do I call it a pitfall for the reader?
Simply, because the reader is at risk of believing that the essayist has, indeed, read the mind of a writer who may be a continent, or several centuries (or both), away, living and writing in a context that the essayist simply does not share.
Now, this can be done for amusement: as for example the devotees of the Sherlock Holmes canon who will defend to the death their claim that Watson -- or maybe Holmes -- was in fact a woman: which is obviously bosh, but it can be fun bosh.
But when it's in a context where it is clearly intended to be taken seriously, a casual reader may simply not have the wherewithal to say (as I said after reading one particular essay), "That's a very interesting, and quite probably useful, way to look at the imagery in those books; but there is simply no way that you can convince me that >writer< actually had that in mind when he wrote those texts."
There are mitigating factors, which mostly come down to two related points: the essayist I'm speaking of was clearly writing for their fellow academics; and academic essays, as a whole, don't tend to fall into the hands of casual readers.
These factors are somewhat abnegated, though, in that the title of the volume could not have been better designed to attract a certain class of casual readers if the editor had set out to do so -- which, I feel certain, she did not.
As to the essays themselves: there was not one that bored me, nor made me think it was bosh or navelgazing. Some of the essays did not, in the end, fully convince me of the main points the essayist was putting forth, but not because the points were trivial or ludicrous.
There are one or two whose relationship to the overall subject of the book seemed tenuous at best. Indeed, there is one where, if a few passing mentions of the Inklings were removed, the integrity of the argument would not be harmed in the least.
Although I am not of the book's target audience, it was, for me, an interesting and enjoyable, even sometimes quite enlightening, if quite slow, read. If (like me) you have been immersed in Inkling lore for many years, though not at a fully academic level, you mighth enjoy it. But I cannot, in good conscience, recommend it to a casual reader who is fascinated by The Lord of the Rings and the Narniad, and also likes tales of King Arthur and the Round Table.