I love his "Concord" piano sonata (No.2, I think) with its themes evoking New England. Ives also wrote a book to accompany this piece, called Essays Before A Sonata which I bought, back in the day, and still have lying around somewhere.

It's now available as a free download from (I think) Gutenberg.org. Thanks for the heads-up about the other link, Keb. I've nabbed it, and will post one to the Essays when I get a minute.

His lighthearted riffs on the tune that he (along with the rest of the USA) calls "America" and the British call "God Save The Queen" are huge fun, but thank God it's instrumental only, so no words to fight about. He was clearly in humor/humour mode when he composed these variations.