Trattado de Glosas
This treatise is part of many in the 16th century, which show you how to apply ornaments to a given melody by giving tiny snippets of melody without ornaments, and then examples of ornaments you could apply to these unornamented melodies in a table-format.
But, Ortiz also shows other improvisation techniques, like improvising over a tenor, improvising in several parts, adding a 5th part to a 4 part piece and more.
A translation is available here:
Solo / Ensemble
Spanish / Italian (German / English in modern edition - see above)
Viola da Gamba, Any
Beginner, (More) Advanced