Elements of musical Decoro

As I describe it here, decoro in music is basic poise and orderliness, getting the right notes with application of a good technique, being well prepared. It is correct pronunciation and good grammar. It is an understanding of the parts and the whole. It is knowledge of methods of expression. It is decorum, appropriateness. It is unity with the character of the material.

It is not the end in itself, but it is a necessary beginning.

The resources listed below are introductions to decoro in Baroque music and harpsichord/organ/clavichord, since those are my specialties. However, the broader principles of decoro, sprezzatura, and grazie apply to any type of performance, not only to music.

Expression (articulation, phrasing, gesture)

These books give an excellent introduction to Baroque expression. One should then continue with as many other historical treatises as possible: and not only for the instruments played by oneself.

Quantz, Johann Joachim. On Playing the Flute. Berlin, 1752. Translated by Edward Reilly. Northeastern University Press, 2001; ISBN: 1555534732 (Essential for players of any 18th century music, not only flautists!)
Harnoncourt, Nikolaus. Baroque Music Today: Music as Speech. Translated by Mary O'Neill. Amadeus Press, 1988; ISBN: 0931340055
Carter, Stewart, ed. A Performer's Guide to Seventeenth-Century Music. Schirmer Books, 1997; ISBN: 0028704924 (19 essays and a general bibliography)

Keyboard fingering

These modern books about historical fingering lead the reader to many early sources as well:

Troeger, Richard. Technique and Interpretation on the Harpsichord and Clavichord. Indiana University; ISBN: 0253204089 (Out of print but he's currently working on a second edition.)
Soderlund, Sandra. Organ Technique: An Historical Approach. Hinshaw Music; ISBN: 0937276006 (Applicable to all keyboard instruments, not only organ.)
Ferguson, Howard. Keyboard Interpretation from the 14th to the 19th Century. Oxford; ISBN: 0193184192