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.
Lastly, if the work was published between 1923 and 1963, you must check with the U.S. Copyright Office to see whether the copyright was properly renewed. If the author failed to renew the copyright, the work has fallen into the public domain and you may use it.