The Oxford History of Western Music, Pt. 13

Although the circle of fifths and its modulatory properties had been theorised in the sixteenth century, it was Arcangelo Corelli who imported the theory into the practical domain. Instead of using continual "perfect" fifths, a single diminished fifth kept the cycle within the diatonic realm, and thus allowed each scale degree to potentially act as a harmonic root (as opposed to being defined by its melodic resolution tendencies).
The result was that Corelli, who only wrote instrumental music, could give the sort of formal structure to instrumental music that vocal music derived from text. Thus Italian instrumental music in the 1680s exhibited a fully elaborated tonal system; pieces began and ended in the tonic key, travelled to the dominant (V), and possibly visited other keys close by on the circle of fifths. What was unique about this use of the circle of fifths was that it played a global, structural role.
The extravagance of fifth-related harmony was often counterbalanced by descending melodic sequences, which created smooth voice leading through the sevenths of thirds of the harmony. In contrast, ascending sequences allowed composers to modulate to relatively distant key areas via what we now call "secondary dominants," setting up the inevitable "unravelling" of harmonic tension via the circle of fifths.
For composers, they now had a way to create tension and release over a large structure without relying on text. Listeners came to listen to these pieces differently; the momentum created by these harmonic structures required closer attention. Their attention was rewarded in experiencing the waves of tension and release, or implication and realization, crafted by composers. Hence an important step had been taken towards instrumental music's autonomy.

The Oxford History of Western Music, Pt. 12

Pg. 173, vol. 2:

Speaking of Opera Seria.

"The highest arbiter of taste and practice was the ruler or patron; next in order of clout came the audience; next the singer; next the librettist. The composer was there to serve them all."

It's interesting to me that we know think of the arbiters of taste in almost the opposite order. Such ideas are the effects post-enlightenment education. Perhaps the most pertinent idea here then, is that much of the music we consider Art, and many of the techniques used in most of music we listen to, comes from a time when the composer was very far down the ladder of power.

The reception of Opera itself in the early 1700s is testament the place music occupied in society. The only times audience members were really required to pay attention was when the king or patron, who the opera also presented as a symbol of all that is good in the world, was in attendance. Otherwise, people were free to talk, wander in and out, play cards, and relieve themselves in chamber pots. Most audience members also new all of the operas extremely well, meaning they could pop in or out whenever they pleased depending on when their favorite arias or singers were on.
Singers themselves thus needed to draw the audience in with virtuosity, improvisation and diva-like behavior. Caffarelli, a virtuoso castrati of the day, was arrested for refusing to sing, mocking other performers, and practically molesting a female singer on stage, only later to be reinstated by the opera company because he was the public's darling.

So, singers behaved, musically and personally, much more like pop singers do today, and the reception of music was more akin to the way we watch television than the way we sit in an opera house.

Elliott Carter, improvisation and metric modulation

The Oxford History of Western Music, Pt. 11

The first appearance of pieces exhibiting the title "concerti" are by Andrea Gabrielli from 1585, and might be described as Masses for voices that are organised into antiphonally organised groups such that their contrasting timbres and musical characters form the main thrust of the work.
That only some of the voices are marked "a cappella" suggests that the other "choirs" are actually played by instrumental ensembles. Thus the earliest use of "concerto" suggests, simply, voices and instruments.
Ten years later the Bolognese organist Adriano Banchieri's piece Concerti ecclesiastici a otto voci was published including a seperate part for the organist that contained a harmonic "reduction" of the many vocal and instrumental parts. Accompanied songs soon after were published with an even more streamlined organ part, called the basso continuo.
The basso continuo was not "invented" at this time. Simply, publishers were now documented a practice that organised had long used in their "oral" tradition.
Interestingly, this new development gave rise to backlash among some organists who saw the published, "fully-realised" basso continuo part as an easy way for the lazy to play something that previously only the studied could.

The Oxford Histroy of Western Music, Pt. 10

The Madrigal (music set to secular poetry) became the hotbed for music experimentation in the sixteenth century. With Ars Perfecta composers such as Josquin streamlining composition to as perfectly express holy texts as clearly as possible (although not without incredible craftsmanship and feats of "composition"), composers used secular texts to create links between the expression in text and music.

Hence the representation of text became the challenge for many composers, and resulted in many different forms of radical experimentation in terms of texture, melody and harmony. Luca Marenzio's Solo e penso. Is the first example of the entire chromatic scale in a piece of music; it begins on F-above-middle-C and proceeds, by semitone, to cover the range of a major 9th, before descending again. Some semitone steps were diatonic, that is, between the 3rd and 4th or 7th and 8th degree of an ionian scale, or were chromatic, that, spelt using a different accidental on the same letter name. The text (Alone and thoughtful I pace the most deserted fields/with slow and heavy steps) is expressed through this mono-rhythmic and mono-intervallic soprano melody. The resulting harmony, if we were to use a system of harmonic analysis based on the 18th and 19th century, is unconventional. The fact of the matter is, however, that this harmony arises out of Marenzio's want to give life to text, rather than experiment with harmony for its own sake.

The use of the complete chromatic scale also suggested a refinement of the tuning system to equally space the 12 semitones through the octave while maintaining the 2:1 ratio of frequencies between octaves.