MLA
Luther, Martin, 1483-1546. Ad Serenissimu[m] Principem [et] D[omi]n[u]m Do. Carolum V. : Rho. Caes. Impe. Aug. Hispaniarum rc̄. Rege[m]. Archiducem Austrie rc̄.. XXXX. Richard C. Kessler Reformation Collection. Pitts Theology Library. https://digital.library.emory.edu/purl/39995x6bhf-cor.
APA, 6th edition
Luther, Martin, 1483-1546. (XXXX)[Ad Serenissimu[m] Principem [et] D[omi]n[u]m Do. Carolum V. : Rho. Caes. Impe. Aug. Hispaniarum rc̄. Rege[m]. Archiducem Austrie rc̄.]. Richard C. Kessler Reformation Collection, Pitts Theology Library. https://digital.library.emory.edu/purl/39995x6bhf-cor.
Chicago
Luther, Martin, 1483-1546, Ad Serenissimu[m] Principem [et] D[omi]n[u]m Do. Carolum V. : Rho. Caes. Impe. Aug. Hispaniarum rc̄. Rege[m]. Archiducem Austrie rc̄., XXXX, Richard C. Kessler Reformation Collection, Pitts Theology Library. https://digital.library.emory.edu/purl/39995x6bhf-cor.