If you've seen the movie, the music is one of the things you really notice - which is not necessarily what you'd expect as it has no lyrics or anything, purely orchestral. But anyway, I watched the movie, loved it, read the book, watched the movie again, thought "I love this music!!!!" and went on a desperate search for the CD.
Most definitely the best soundtrack to a film I've ever heard and simply way up there with the best music period. The "To the Stars" theme is - magic. Moving, beautiful... listen to it, I don't know what else to say. It fits completely with the story of the movie, although it stands alone fine too, but you can pick out character's themes and little motifs. Apart from the practically heartbreaking main theme, there are all these little lighter pieces too, and it all comes together - beautifully.