Debussy wanted to distance himself from what he considered the emotional excess of the Romantics and instead wanted to write music that, through sound, created an ambience similar to that of the Impressionist painters and Symbolist poets of his day.
All books initially copyrighted in the US from 1964 through 1977 have had their copyrights automatically renewed (by law) and the copyrights are still in force. The initial copyright term was 28 years; the renewal was for 67 more years.
However, even if the author died over 70 years ago, the copyright in an unpublished work lasts until December 31, 2002. And if such a work is published before December 31, 2002, the copyright will last until December 31, 2047.