as far as zazen, the two books i recommend are 'zen mind beginner's mind' by shunryu suzuki and 'the three pillars of zen' by philip kapleau roshi.
both are excellent.
in regards to vipassana, i would simply recommend the
traditional therevada buddist writings. if you're interested, i think it would be best to take a
free course.
metta meditation is a part of vipassana, specifically practiced at the end of each sitting to share goodwill with all. it is actually the last of three stages, practiced after breath observation to calm the mind and then the balanced observation of changing sensations.
such meditation expands awareness and understanding of THIS.
i would emphasise the value of continuing actual practice as opposed to doing too much reading about it, although reading can be helpful.
by the way, i'm a musician. i'll send you a
free cd if you like!