Wechsler, Michael G., Evangelium Iohannis Aethiopicum, CSCO 617, Leuven: Peeters, 2005.
with an introduction, mentioning the kind of text Eth John reflects (mixed, as you might have guessed). The main corpus is a critical edited text of Eth John, based on the earliest Ethiopic MSS. Besides, an additional chapter lists minor variants, and another one has suggestions for the NA27 text, in Greek (for that, you do not need Ge'ez!).
It seems to be a thoroughly edited work, which will be very helpful in the future editions of the IGNTP / ECM Gospel of John volumes. In the foreword, he acknowledges, besides others, the help of R. Zuurmond (who edited Eth Mark & Matthew) and of God (who, according to him, seems to have been somehow involved in the production of the Greek Vorlage ...).
Anyone who wants to know more about the Ethiopic Bible: Take a look at the Encyclopaedia Aethiopica, Vol. I and II are now out (Harrassowitz, Wiesbaden). Look for the article "Bible" in Vol. 1. Very helpful.