Verlyn Flieger's book "Splintered Light" is a scholarly analysis of Tolkien's "The Silmarillion" and "Lord of the Rings", with most of the emphasis on the former. Flieger also comments on Tolkien's own scholarly works, "The Monsters and the Critics" and "On Fairy-Stories".
"Splintered Light" is relatively easy to read, but its subject matter might be to narrow for the general audience (including most fantasy buffs).
I was struck by two things when reading it. One was Tolkien's strong pessimism. Middle Earth is the product of a cosmic fall in several steps, each new step making the world even darker and more evil. Death is a "gift", and the only way to Light is through Death and Darkness. This almost morbid pessimism was apparently connected to Tolkien's depressive private personality.
The other thing that struck me was the strong "paganism" of Tolkien's works. While Tolkien claimed that he was hiding Christianity for some kind of tactical reasons, and that his work was Christian in spirit, it does seem to differ even from the spirit of Christianity on several important points. Thus, humans are created in an already fallen and mortal state in Tolkien's universe. Evil isn't a privation, but seems to be a real substance, dualistically confronting the Light. The outcome of the cosmic struggle is always in doubt, and Flieger perceptively points out that Frodo actually *failed* in "Lord of the Rings", succumbing to the evil powers of the Ring.
Perhaps Tolkien didn't see any contradiction between this and his Christian faith, but it's a common criticism of "Lord of the Rings" that the evil characters (and evil itself) is somehow more convincingly described - and more potent - than the forces of good. (As a side point, Swedish TV once interviewed a Satanist to claimed that his view of Heaven resembled Middle-Earth!) On a more trivial level, there is the implicit polytheism of the creation story in "The Silmarillion", with God (Ilúvatar) showing a kind of Platonic forms to a host of lesser deities (Ainur), who then creates Middle Earth. Some of the creators, the Valar, eventually descend and find a place of habitation on Middle Earth as a kind of Olympic deities.
Flieger believes that Tolkien was to some extent influenced by Owen Barfield's book "Poetic Diction" and more broadly by Barfield's view of language and myth. Perhaps, but personally I find the differences more striking. Tolkien did regard the poet as a sub-creator, but the world created by the poet seems to be an illusion, a kind of faint memory of our existence before the Fall. It's difficult not to view Tolkien as a hopeless escapist. Redemption is a matter of faith, and awaits us in a very distant future. By contrast, Barfield believed in an ongoing cosmic evolution, so in his scenario the sub-creators are quite literal. By changing the consciousness of humanity, poets can change reality itself, since reality is a "collective representation" of our cultural consciousness. Like his mentor Rudolf Steiner, Barfield believed that the "fall" of humanity was a necessary part of the cosmic process. Indeed, we couldn't exist as fully-developed, self-conscious individuals without an initial estrangement from Nature and the spirit-world. This state is quasi-dialectically transcended at a later stage of evolution, when we return back to Nature and Spirit, but with our self-consciousness intact. In Tolkien's scenario, there doesn't seem to be anything positive about the Fall at all. It seems to be a never-ending descensus into darkness, with a supernatural solution only at the very bitter end.
Although both Barfield and Tolkien were anti-modernist and retro-romantic, Barfield's perspective is nevertheless a strange form of optimism, rooted in Steiner's occult teachings of Anthroposophy with its upward-moving cosmic cycles, while Tolkien's perspective is a traditional Christianity with a strong pessimistic and "pagan" spin á la Ragnarök. Perhaps Tolkien did read "Poetic Diction", but he clearly never understood where Barfield was coming from...
Finally, a disclosure. I've read LOTR, but I haven't read all of "The Silmarillion". Frankly, have you? However, I read sufficiently much to catch the main lines of Flieger's arguments. As already indicated, "Splintered Light" is probably too narrow for the general reader, but as a closer analysis of Tolkien's mythology, it does deserve four stars.