Flieger's Tolkien analysis was recommended to me last year, and this is her most popular book (also seems to be the only one available in ebook format). I found it very interesting. I was less convinced by her strong thesis, that Tolkien's core message is to do with splintered light v darkness, but rather more so by her incidental detail, that when choosing words Tolkien was very aware of their Indo-European roots and some of his choices of phrase particularly need to be understood in that light. She does have some good evidence, notably the Silmarils and the undoubted intellectual and personal links between Tolkien and Owen Barfield who had ideas along the lines, but I think there is so much going on in Tolkien's work taht it can't really be reduced to just this theme (and I thought her treatment of Tolkien's own personality was a bit awkward).

It's rather dated - the first edition is from 1983, and perhaps is an attempt to explain the Silmarillion; the second edition, from 2003, draws rather less on the History of Middle-Earth, which had all been published by then, than I would have expected. Also absent is any mention of how the light/dark good/evil dichotomies might be read in terms of Tolkien's attitudes to race, which feels like a big omission. Still, I'm convinced enough to order A Question of Time.