I'm making available on newbyz.org a book called "Guide to the Music of the Eastern Orthodox Church" by N. Lungu, G. Costea, and I. Croitoru, translated from the original Romanian into English by Fr. Nicholas K. Apolstola.
The main purpose of this book is to teach Byzantine notation by showing how it is transcribed into Western staff notation. Given the paucity of books on Byzantine music theory in English, I see this as an invaluable text.
In addition, there are some forewords in this book that serve as excellent brief essays on the purpose and use of traditional Psaltic music in the Orthodox Church. It is well worth downloading this file just to read these essays.