Holtz, Traugott. Die Christologie der Apokalypse des Johannes. (TU, 85). Berlin, 1962.
Holtz’s Christologie is one of two or three in depth researches in the Book of Revelation. Holtz accepts results from historical-critical works with regard to sources, redactions etc., which, in my opinion, invalidate a number of his conclusions.
This book is nevertheless very valuable.