If it's a book of songs, you might want an ISMN. (M for music) I'd never heard of them until Jack Crawford told me about how he got one when he re-published the "Red Book" of Sheffield carols. Try emailing jack (at) forgefolk (dot) com - he might have some spare...

Folkiedave a.k.a. Dave Eyre will be along soon to help out, I'm sure, and he will definitely know.