The main selling point for most fans would be the rare photographs. I would imagine that nearly all, if not all of these have been reprinted again in other PomPress books since the volume's original publication in 1996.
Some appear in other PomPress books, but many, if not a majority, are unique to the Music Book. The cover photo is one that was subsequently reprinted (in the Movie Book) - but it's an inferior print of it - and it's not in color. And for anyone who's a fan of NoDS, that cover photo alone is well worth owning the Music Book because it's a spectacular photo, especially in color.