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 principle it's very difficult to know. In practice, it's possible to find texts that are in the public domain, which you can use. I am not a lawyer, and copyright law is very complex. As a general rule, in the USA, if something was published before 1923, and was first published in the USA, it is out of copyright.