>Start with Tolkien's essay "On Fairy-Stories" in The Tolkien Reader, which
I'm sure is still available in paperback (I hope so, since I'm about ready
for a new one). Then, in the same book, move to "Leaf by Niggle" for a
delightful use of the ideas. "On Fairy-Stories," in fact, includes the
poem which Tolkien says he "...wrote to a man who described myth and
fairy-story as... 'Breathing a lie through Silver.'" The man he wrote it
for was C.S. Lewis, and although the poem isn't given a title in the essay,
it usually goes by "Mythopoesis."
The title of the poem is "Mythopoeia". Only a few lines are quoted in "On
Fairy-Stories". For the complete poem, don't go to _The Tolkien Reader_,
but instead, find one of the editions of _Tree and Leaf_ beginning in 1988
that say "including the poem Mythopoeia": in these the complete poem has
been added to the volume.