off topic - asking for music (voice) teaching help

My daughter has decided (after a brief flirtation with guitar lessons )to attempt studies in voice.

I was wondering (since there are lots of teachers on this list) if someone had a recommendation as to a book or series to purchase. I would be doing the teaching - but I tossed my voice class books long ago.

It needs to be relatively inexpensive because the likelihood of her sticking with it is not very high.

FYI - she is 13.

Thanks,

Chris