The Division Violist
Beside being a treatise on how to perform on the viola da gamba, there is a lot of information on how to improvise on grounds. Interesting is his idea of "points", a certain motif that you use throughout the variation. He also talks about ensemble improvisation.
There are also written out examples of entire ground variations and the in the renaissance period typical table format of ornament variants.
German translation and modern edition:
Solo / Ensemble
Type of Thing you Learn
Diminution, Diminutions, Passaggi, Ornamentation, Grounds, Ostinato
Viola da Gamba, Any
Beginner, (More) Advanced