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.
For any work created from 1978 to date a copyright is good for the author's life, plus 50 years, with a few exceptions such as work for hire which is owned by the one commissioning the work for a period of 75 years from publication. After that it falls into the public domain.