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.
You should also be on the lookout for first printings of a A Fly Went By and Book of Riddles. The first twelve Beginner Books were published between 1957-to-1959 and should be on your 'watch for' list. First editions are easily identifiable, as all but Cat In The Hat state 'First Printing' on the copyright page. We've documented first edition identification points for Cat In The Hat on our blog.