[j. d. worthington]

Asmiley: You can find copies of the Letters online (even hardbound) for very reasonable prices... I just got a replacement of the first American edition (later printing) for about $5, in very nice shape. And, as Nesa was mentioning elsewhere, I'd also recommend the Father Christmas Letters, both as enjoyable in themselves, and because it can also give insights to how he was working things through with these topics, from a different angle.

Frontierzone: Yes, the Foster is a good quick reference guide to a lot of things, though it doesn't cover much of the material published later on; still, a handy and fascinating book. There's also The Tolkien Companion, by J. E. A. Tyler, which has some interesting aspects, and A Tolkien Bestiary by David Day, which is also interesting as a reference work in its own right (not to mention for some beautiful artwork). You might also be interested in some of the things in Carpenter's biography of Tolkien (I can't recall whether you've mentioned that one or not) as well as his book on The Inklings.