According to Neumann, an “exhaustive, excellent” monograph on the subject is Erläuterter Katalog der deutschen Flussgold-Gepräge (the "Illustrated Catalog of German River Gold") by Franz Kirchheimer, which is available for loan from the American Numismatic Association library.
The work provides a detailed, condensed list of the various issues from the three major rivers, details that are also scattered throughout listings in Gold Coins of the World by Arthur L. and Ira S. Friedberg.
Both ducats in the recent auction were struck for Bavaria, which had just taken over the city of Speyer in 1816. Two slightly different skyline views of Speyer along the River Rhine appear on the reverse of the coins, the Speyer Cathedral at the center of the scenes. Both coins carry the date of issue in Roman numerals and a Latin reference to the origin of the gold, EX AURO RHENI on the reverse.
The 1830 ducat, cataloged as Friedberg 275, shows a young head of Bavarian King Ludwig I on the obverse. The 1842 ducat shows the older head of Ludwig I.
Though perhaps little known, the coins are interesting artifacts of gold panning along the Rhine in 19th century Europe.