Crux et Cithara is a 1983 book, completely unavailable, edited by Robert Skeris in honor of the 70th birthday of Johannes Overath. It contains many important pieces that I look forward to reading now that the technical side of things is done. It is right here.
Among the pieces is J. Ratzinger's Theological Problems of Church Music, which has also been completely unavailable (to my knowledge). So it is extracted as a separate file. If anyone wants to extract the text and send it to me in HTML, please do, and we can put it up at MusicSacra.com.