Antonij,
There are several studies published in English, either chapters in some books or articles in periodicals (I have in mind Sandra Martani's article, "The Theory and Practice of Ekphonetic Notation in the Manuscript Sinait. gr. 213", in Plainsong and Medieval Music, 12, 2003, an e-periodical not accessible free of charge). If you want, let me know your desiderata through private email.