I like this idea, & wish I felt qualified enough to contribute as well as Mark. I have read a lot of the recent books on Tolkien, & found most of them trivial, unfortunately. Also, as I'm posting from work, I don't have the relevent volumes to hand, so I can't post all the details Mark asked for. But...
One recent favourite though is: Secret Fire: the Spiritual Vision of JRR Tolkien
http://www.secondspring.co.uk/fantasy/secretfire.htm
This is a book by a Catholic author which goes deeply into the symbolism of Tolkien's work, but unlike so many similar works, doesn't use it as a way to try & convert anyone. I'd say its the best intro for anyone wanting to know how Tolkien's beliefs were 'translated' into his fiction.
I've just begun reading Matt Dickerson's 'Following Gandalf', & am finding it quite fascinating, but I won't speak too soon!