Which composer wrote the microtonal-leaning Concord Sonata?

Charles Ives, who wrote it between about 1911 and 1915, published it himself in 1920 with a book of essays explaining it, and gave copies away because nobody would buy it. It contains a passage requiring a board of wood about fourteen inches long to depress a cluster of keys.

He sold insurance. Ives ran a very successful agency, essentially invented modern estate planning, and composed at night and at weekends for decades - so a body of work full of dissonance, polytonality, quarter-tones and simultaneous marching bands was produced by a man with no interest in a musical career and no audience to lose. Much of it was not performed until he was old or dead; the Concord Sonata got its first complete public performance in 1939, and the Third Symphony won a Pulitzer in 1947, over forty years after it was written.

The dates are disputed. Ives revised constantly and dated manuscripts loosely, and a 1987 study argued he had systematically backdated works to appear more of a pioneer than he was - which matters, because the claim is that he anticipated European modernism in rural Connecticut. The counter-argument is that the revisions are documented and the misdating mostly careless. Either way the sound is his: he grew up hearing his bandmaster father experiment with two bands playing different tunes as they marched past each other.