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.
In the public domain due to copyright expiration. 1923 through 1977. Published without a copyright notice. None. In the public domain due to failure to comply with required formalities. 1978 to 1 March 1989. Published without notice, and without subsequent registration within 5 years. None.