Letters of Note has pointed out a Tolkien rarity, his reply to Elise Honeybourne’s 1967 fan letter summarizing for her why he wrote The Lord of the Rings:

I wrote The Lord of the Rings because I wished ‘to try my hand at a really long story that would hold the attention of readers, amuse them, delight them, and at times maybe excite them or deeply move them.’ As a guide I had only my own feelings for what is appealing or moving; and it has been a great pleasure (and a surprise) to find that so many other people have similar feelings. 

A transcript of the text is here. The scanned pages of the letter are posted here.

[Thanks to David Klaus for the link.]