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.
If the work was published without a notice or if the copyright has expired, it is not in the public domain. Works published between 1964 and 1977 - When published with notice, it has copyright protected for 28 years for first term, with an automatic extension of 67 years for the second term for a total of 95 years.