L’Art de préluder sur la harpe
This was written to follow his harp method. More information about this method can be found here (harpmethods). In terms of improvisation it seems to only discuss ornamentation (Arpeggios, Trills, Tremolos, Bisbigliandi, Ornaments (pages 7-9), and not how to actually use these modulatory passages in your preludes.
A resource for the harp, which shows in regular notation how to modulate (change) from almost all keys to another key for about 17 key changes. Here you can the beginning of how to move from a flat major to C major, A major, D major, B flat major, etc. The fixed Do system is used, so La is an A, Si is a B. They still use the Ut instead of Do, which is a remnant from medieval music, that was still in use in renaissance music, became less common in baroque music.
What could you use this for? Basically it's a modulation-flip book. For this style and your enrichment, if you don't know yet from harmony or want a refresher, this could be a nice way of learning how to do these modulations. Obviously there are other ways as well.
At the end he shows how one might do this to (all) 23 keys from C major and minor in a basso continuo notation.
Solo / Ensemble
Louis Charles Ragué (1744 — fl.1793)
Type of Thing you Learn