Ratner
Chapter 11 of Harmony: Structure and Style is on dominant sevenths and the tonic 6/4. Chapter 12 is titled "VII7 and V9--The Major Sixth in Dominant Harmony"; it ends with a helpful summary. Here that is, with further quotes and my comments:
1. The major sixth in dominant harmony appears in two chords, the V9 and the VII7. [Note that Ratner uses the non-distinctive capital letters of scale-step theory. Since he is writing only about the major key at this point in the book, we know that VII7 is the same as viiø7.]
2. The sixth degree adds a strong characteristic element of color to dominant harmony. [He is referring mainly to later 18th century music, where the supporting harmony is viiø7: his first two examples are from Mozart and Beethoven. But a brief excerpt from Brahms, Symphony no. 1, II, has a V9 in first inversion--see below--though in typical Brahmsian fashion it sounds as much like an accented neighbor chord as an independent ninth chord.]
3. Usually, the sixth degree appears in the uppermost voice, since it possesses marked melodic value. [In the main text he says ^6 is best in the top voice "where its melodic tendency will be realized in a salient manner."]
4. The sixth will tend to resolve downward to the fifth degree, before the dominant harmony moves to tonic or at the change of harmony. [This "tending to resolve downward" comes from the idea of ^6 as a neighbor to ^5. His options cover my internal and direct resolution categories, respectively.]
5. The sixth may leap downward to another tone of the dominant harmony in a chordal melodic figure, relieving the sixth of the need to be resolved directly. [His point about this in main text, though, is that ^6 makes a very effective "melodic apex," after which comes a fall to the cadence or other resolution.]
6. In a 6-7-8 pattern the sixth moves upward to the leading tone, and the V9 is first struck without a leading tone. ["Occasionally, when one voice moves 6-7-8, the harmony momentarily becomes V9 with 6 supplanting the leading tone at first. The musical effect of this can be quite poised and elegant, as shown below."]
7. V° and VII' are usually used within a phrase, except in cases described in summary points 5 and 6.
Sessions
Of all the books surveyed in this series of posts, Session's Harmonic Practice shows the most obvious evidence of an experienced and practically- minded composer. "Frozen" accessory tones are of two types: "Ninth, Eleventh, and Thirteenth Chords" and "Substitute Functions." He offers no method of derivation, just says the ninth chord is "generally classified as a basic chord type," later adds "so-called" to first mention of elevenths and thirteenths, and ends the section with the statement that "in the overwhelming majority of cases, these chords are used on the dominant degree, but use on other degrees is also possible."
The ninth can be resolved internally because the root is already present (he shows both V9 and I9) and thus the ninth can be left by leap internally, but the same cannot be said of the seventh.
As to inversions, they "are extremely varied in harmonic effect, even in the case of different positions of the same inversion, which . . . if they are played in alternation seem to denote a genuine change in harmony." Here is the second inversion of C: V9.
