I am sitting down to practice. I open the case and take out my instrument, a classical guitar made from the door of a Spanish church. I strike a tuning fork against my knee and hold it to my ear, then ...

At 8, Glenn Kurtz was a prodigy. At 19, he was a promising classical guitarist at the New England Conservatory of Music. At 25, he was a professional musician playing in Vienna. But something was ...