I have just read the book. Compared to the book, I find the film decidly better. The director & the novelist worked 2 years on the screenplay and we see a very good result of their work on the screen! Juraj Herz is a genius!
shareOne thing our teacher pointed out is how often music is used. It's not so obvious in the book, though there also is a lot about music - but in the film one can see what type of music all the mentioned works are: tragic, dark.
One important thing is, in my opinion, that the screenplay was written by author of the book.
"There is Robinson, alone on lone island - but they will marry."
The screenplay was written by the author and the director for 2 years!
The music is very important in this film, you are right.