I have found "Light science and Magic" from Fil Hunter and Paul Fuqua to be a great reference on lighting.
Portrait is only a small part of it, and there are probably more specialized books that offer more examples of what can be done, but this reference provides very solid basics on lighting and might be a good place where to start from.
It doesn't really cover the specifics of digital shooting though, but 95% is still completely relevant.
Regards,
Bernard