If you are interested in 18th-century music, you'll want to be familiar with the three great mid-century treatises. They are written for three different instrument-types, but they broadly agree and it is easy to apply their advice to the harp or other instruments.
Leopold Mozart's 1756 treatise on the Violin has a lot of information relevant to Wolfgang Amadeus' Concerto. The German original is here, An English translation is here.
Make sure your flautist has studied Quantz's 1752 treatise on the Flute, and read it for yourself too, here. You'll find an English translation in a good music library, or on Amazon, here. There are two examples of how to play an Adagio: one shows how to vary the dynamics note by note, according to the intensity of dissonance/resolution; the other shows how to realise ornamentation, varying the dynamics note by note, even within each ornament.
CPE Bach's 1753 treatise on Keyboard instruments, extended in 1762 with a section on Continuo, is here. Your library should have an English translation, or find it on Amazon here.
Mary Oleskiewicz's essay on the CPE Bach Sonata discusses the work in the context of period instruments, the Italian triple harp, the French 'single-action' pedal harp, with some unexpected suggestions for performance options. Read it here. Ann Griffiths' article on the Sonata is here.
There is a wealth of detailed information about Ornamentation in the three great treatises and in late-18th-century Methods for harp. As a starting point, I've summarised all that information into Three Commandments.
1. Ornaments are on the beat (not before).
2. The auxiliary note is usually long
3. Most ornaments decrescendo
There are some ornaments that start fast (e.g. upward slides) contrary to my 2nd Commandment. Quantz and CPE Bach give complex dynamics for individual notes within ornaments (way beyond my 3rd Commandment). So treat these Commandments as a starting point, and go to the historical treatises for more detail.
18th-century Methods admit that trills are difficult on the harp (even more so, on high-tension modern harps). Don't try for too many iterations, rather concentrate on the Three Commandments.
Beat Wulf 's highly informative Single Action harp Timeline is here.
Irish harps (medieval/renaissance, 17th-century chromatic, 18th-century) Ancient Gaelic repertoire, Lawes Harp Consorts, Carolan
A series of videos on historical techniques for Irish harp starts here.
A series of videos on Irish ornaments starts here.