Beagle, Peter S., "Tolkien's Magic Ring," in J. R. R. Tolkien, The Tolkien Reader, Ballantine Books, 1966, pp. ix-xv.

An excellent, if non-technical and short, introduction
to The Lord of the Rings by another celebrated writer
of fantasy.

Rosebury, Brian, Tolkien a Critical Assessment, St. Martin's Press, 1992.

An excellent extended study of Tolkien's style, and an
antidote to a lot of extremely bad criticism.

Shippey, T. A., The Road to Middle Earth, Harper Collins, 1992.

One of the finest pieces of Tolkien criticism yet
written, it is unsurpassed for the sources of The Lord
of the Rings
and the influence of Philology upon
Tolkien's work.

Tolkien, J. R. R., "On Fairy Stories," in The Tolkien Reader, Ballantine Books, 1966, pp. 3-82.

Written while Tolkien was beginning the Lord of the
Rings
. It is a critical theory and justification for the
trilogy.