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 example, the copyright date that you see on the inside of the book jacket likely coincides with the date of formal registration or the date that the work was finished and submitted to the publisher, but not the time that the book was first written, which is when the book first earned its copyright.