Italienische Diminutionen / Italien Diminutions
This is the most expensive book I ever bought, but it's well worth it. The introduction gives a clear, concise and yet detailed overview of the execution of diminutions on different instruments and their historical background.
More amazingly it is a collection of all vocal pieces with more than one ornamented version from that time (1553-1638) and puts them all in the same score. This way you can literally compare word per word, bar by bar, which ornaments which musicians used where in comparison to their contemporaries.