I recently found one of my favorite resources on Baroque performance practice and ornamentation on IMSLP:
To further explain what it contains, here is a description of this text from Amazon.com:
Johann Joachim Quantz’s On Playing Flute has long been recognized as one of the most significant and in-depth treatises on eighteenth-century musical thought, performance practice, and style. This classic text of Baroque music instruction goes far beyond an introduction to flute methods by offering a comprehensive program of studies that is equally applicable to other instruments and singers.
The work is comprised of three interrelated essays that examine the education of the solo musician, the art of accompaniment, and forms and style. Quantz provides detailed treatment of a wide range of subjects, including phrasing, ornamentation, accent, intensity, tuning, cadenzas, the role of the concertmaster, stage deportment, and techniques for playing dance movements. Of special interest is a table that relates various tempos to the speed of the pulse, which will help today’s musicians solve the challenge of playing authentic performance tempos in Baroque music.
The whole text is available from IMSLP in the original German as well as in French translation. There is also an English translation of Chapter 13 only, regarding ornamentation, with the wonderfully florid title: Easy and fundamental instructions whereby either vocal or instrumental performers unacquainted with composition, may from the mere knowledge of the most common intervals in music, learn how to introduce extempore embellishments or variations; as also ornamental cadences with propriety, taste, and regularity, translated from a famous treatise on music, written by Johann Joachim Quantz, composer to his Majesty the King of Prussia.
That florid title is not too surprising, given that this public-domain English version was published in 1780. Actually, the 18th-century English usage, typeface, and music notation of this edition are a rather challenging read for me, but there is a more modern English translation of Quantz’s text available for purchase from Amazon.com and other sellers if you find yourself in the same boat.