Having reached this general conclusion, there are two options for your further education, assuming you're not taking a university course requiring you to do specific readings - which, of course, would be a highly recommended starting point if that option is available to you, but which will inevitably also come to an end at some point. The first option would be to read some of the modern works on interpretation. There are many in English, many by highly respected performers and scholars, including Bruce Dickey, Bruce Haynes, and Frederick Neumann, just to name a few - I won't go into the details here, but I expect to review some of their titles in the near future.
These works are useful, because they collect together a lot of the treatises of the period, translate them where necessary, discuss them, and to an extent interpret them as well. But there will also be times, whether for academic reasons, due to uncertainty about interpretations, or simply to go into more detail, when you may want to go to the original sources yourself. Sometimes this will mean relying on the translations of others - most treatises are originally in French, Italian, or German, with only a handful in English. But on some occasions, the originals were in English. And in these cases, it's often possible, and desirable, to go back to the original.
One such treatise is Francesco Geminiani's The Art of Playing on the Violin, published in London in 1751. It's readily available in a facsimile edition - a direct copy of the original, which is surprisingly readable once you realise that f and s look just about the same in old English. Editions such as the one I have linked to provide only the original text, with no introductions by modern scholars or interpretive notes on each page. This can be a positive or a negative, but one thing is for sure: it saves you an awful lot of money! If you're comfortable enough to tackle the bare bones, you can get them for a steal.
So, let's talk a little about this particular treatise, starting with some context. I was lucky enough to have a lecture on the interpretation of his treatise from the magnificent Peter Walls, a Corelli scholar in New Zealand, so much of this comes from him. Geminiani was Italian, studied under Corelli, one of the greatest violinists of the period, lived in France for some time, but spent most of his later life in London.
This mobile life, among other issues, causes a number of issues of interpretation in itself. Let's have a little think about this.
Upon reading his Wikipedia article, one might assume that, having learned from Scarlatti and Corelli, Geminiani's style was thoroughly Italian. But it is well-known that he was absolutely obsessed with French style in the later part of his life, and his later compositions reflect this change, though the Italian style of Corelli, which Geminiani championed as his student, remained in vogue in England. What does this mean for our interpretation? Well, it means that with regards to style, we have to take it with a grain of salt, and interpret his suggestions using the knowledge we already have.
Very little of Geminiani's treatise is actually text: only nine pages, in fact, the rest being musical examples and exercises, the text primarily acting as explanation for them. Many of the explanations are quite technical for violinists - where to place your fingers in particular exercises, for instance - but three sections in particular may be of great interest to everyone. One of these does deal with something for the violinists, specifically how to hold the instrument; another is a table of ornaments, including detailed descriptions, not only of how and when to execute them, but of their emotional meaning.
But I will begin with the third, which is actually at the beginning. He start by saying that the goal of the violinist, and indeed of the art of music, is to 'rival the human voice', adding that those who use their instruments to produce other sounds, or who are unnecessarily flashy or showy, belong to the realm of 'legerdemain' - that is, deception, or trickery. He says that it is our job as musicians not only to please the ear, but to 'express sentiments, strike the imagination, affect the mind, and command the passions'. This is a central message of many period authors, but Geminiani's is one of the most clear and direct statements of this general philosophy of music.
The way of holding the violin is quite simply fascinating; even most specialist period violinists today do not hold it by this method, though I do know a couple in Australia who do, and in all honesty, they are among the best I know. The issue is that Geminiani asks the player to hold the violin below the collarbone, where there is seemingly nowhere for it to rest. I have heard it explained that this could be to do with the clothing of the day, that the violin could nestle in one's coat to prevent slipping. But Geminiani doesn't mention this, but simply states that holding the violin so that the neck is horizontal to the base should prevent the violin from falling.
Having heard many violinists suggest that it is simply preposterous to play this way, but also having seen such technique employed to brilliant effect, and citing various pictures from the period (such as this one of Veracini), I would make this observation: the necessity for playing with this method is excellent posture. If one stands fully erect, without leaning forward, with a high chest, the violin really does sit there brilliantly. This being the case, it can tell us something generally about the way in which Geminiani expected a musician to hold him or herself: with a proud and upright posture. In this day and age, this may be an excellent exercise for us all!
(For an interesting and well-informed look at various ways of holding the violin, there is an excellent and easy to read essay by Richard Gwilt here.)
Finally, and with most relevance to all early musicians, comes Geminiani's table of ornaments, and their descriptions. There are a few fascinating things to say about the ornaments themselves. He refers to the use of piano and forte, for instance, as an ornament; similarly various devices of articulation, including staccato, separation, and even holding a note - which, he points out, is an important way of making sure the melody does not get too lost among all the beats and shakes. These, of course, are themselves described, being among the most common ornaments.
But perhaps of most interest is his lengthy description of the close shake, better known today as vibrato. Many less experienced performers of early music shy away from vibrato, having heard that it is outright wrong for the style. But Geminiani suggests that it should be made use of as often as possible. However, he qualifies this: it is for the expression of particular emotions, and should be approached differently depending on the emotion to be expressed, whether it be 'majesty, dignity ... affliction, fear' and so forth.
What I personally find most fascinating about his descriptions is the way he connects them to emotions; while some are said just to be appropriate to certain types of movement, others are directly associated with sentiments. The turned shake, for example, expresses gaiety; the superior appoggiatura expreses love, affection, and pleasure; the beat, depending on how it is performed, expresses strength, fury, anger, or resolution. He recommends practicing all of the ornaments regularly, in order to achieve the full range of musical expression.
Overall, Geminiani's treatise is of course most important to aspiring baroque violinists, and would certainly be very helpful to other early string players, but there is much in there for all of us to learn. The exercises of course only work for strings, as they assist with double-stops, left hand movement, and string changes among other things, but his descriptions of ornamentation and of music in general are fascinating on their own. Just remember when applying his teachings that he was a very international musician; if something seems wrong to you in a particular style, you're well within your rights to go with your gut. But however much practical use you do or don't get out of it, it's still worth a look!