Houses of The Holy. A departed from the blues based influenced first albums', to new sound territories like the use of mellotron, never used before in previous albums. Also there a new approach to songs to new harmonies and rhythms never tried before. Reggae based tunes, funk power, atmospheric landscapes and even if you listen carefully you can find a little accapela and doo-woop style in the last minutes of the last album song "The Ocean".
BTW, the album in/out sleeve design by Hipnosis and inspired by the ending of Arthur C. Clarke's novel Childhood's End, is very beautiful.
Just a side note: aside from the reproduction of the lyrics to "Staiway to heaven" only "Houses of the Holy" contained lyrics.
One of my top ten albums of all time. Truly a remarkable record.
http://books.google.pt/books?id=lZslRYfLlK...page&q=&f=false