> Is word frequency changing? How is this measured and analyzed?
One simple (and perhaps naive) method would be to go to google, and do word searches, and record the hit counts. This is, in fact, an interesting way to check spelling: which form is more widely represented in the world of the web ? Take the more widely represented form as the "correct" form.
> Sentences were generally much longer and more complex in the 19th century, did the telephone or some other technology change this?
Yet another hypothesis, just invented by myself, might be that familiarity with Latin language literature caused a tendency towards longer and more complex sentences, and the decreasing familiarity with same caused a decreasing average length and complexity.