From Indonesia, a question about organum

Hello,
Introduce allow me to introduce myself.

I am a member of local Gregorian Chant choir "Schola Cantorum Gregorianum Bandungensis"
We are small community with constant weekly practice. Our member consist about 10+, but every practice only 4-5 members. And we are 3 years old group.
Our choir group is supported by parish pastor. Despite many times rejection by Cathedral, our member zealously still practice Gregorian Chant, seeking refuge in parish by parish. Thanks to God, we got refuge in local parish. We are the only group who sing Gregorian Chant in whole city.

Our Group founded when Summorum Pontificum is issued. We happy heart we try to celebrate Extra-ordinary Mass.
Although our bishop nor support or reject, we finally can celebrate this mass. But only last few months because the priest is old and weak.

May I ask? I got few question about organum technic.
Where I can read about how to sing organum?
We tried strict organum when sing Kyrie from Mass IV, but shy when serve in Church.
I have tried to find any practice book about organum, but I cant find any.
Would you help me?

Thank you very much. Pax!