The new critical edition of the Corpus Juris Canonici (1879-81) was prepared by Friedberg, as was also the Formelbuch des deutschen Handels-, Wechsel-, und Seerechts (third edition, 1894). Alike in his collaboration in the Prussian church laws of 1872 and as an author, he showed himself a champion of state supremacy in ecclesiastical matters, and many of his works deal with this subject in its various bearings. Perhaps the best known of his numerous publications are the following:
Die Geschichte der Zivilehe (second edition, 1877)
Lehrbuch des katholischen und evangelischen Kirchenrechts (fifth edition, 1903)
Verfassungsgesetze der evangelisch-deutschen Landeskirchen (1885, et seq.).