I've just read the Power of the Ring and I found it a really enlightening book - I loved The Lord of the Rings when I was younger - and read it many times, excited by the mythic power of it - but perhaps not really sure why. This book explains how Tolkien's faith (or for atheists, the Christian myth!) informs the book, just as much as Norse mythology and heroic literature do.
For me, to be really worthwhile, a book has to say something about the human condition, and I hope to find something inspiring, which encourages me to feel that there is a human spirit which will work with others to achieve great ends and overthrow evils. The Lord of the Rings does this of course, and Caldecott does a brilliant job in revealing the roots of this creation. He highlights the moral and spiritual struggles of the protagonists, which we can all relate to to a greater or lesser extent, while drawing analogies with Christian ideas and examples. I think any lover of Tolkien would find this book enlightening - as it draws on ideas and ideals that are part of the common culture of the Western world.
I found his appendix "The Film of the Ring" particularly interesting, it provides an extended review of the film, highlighting the changes that Peter Jackson had to make, but showing how they are often very much in the spirit of the book.
As someone who hasn't read the Lord of the Rings for 25 years the Power of the Ring made me want to re-read it and see it in a new way - which is a tribute to Caldecott's insights.