Yet it remains far from clear to me whether or to what extent the language is a factor in creating the culture as distinct from, say, merely its instrument or vehicle. Indeed, I suspect that to frame the question that way is to answer it: Yiddish is indeed interesting, but in large part for its role in service to the larger culture.
Perhaps the best part of the book is the discussion of the long tradition of Jewish religious learning that imbued the shtetl culture. As Harshav suggests, only God speaks in declarative sentences, speaks narrative. Others may question or comment--and questioning or commenting turns into a way of life; the students even learn to argue with God, though he retains the last word, does not seem to take offense.
It is right here, I think, that we locate the idea (not specified by Harshav) that the key to shtetl religious life is not knowing Torah but the study of Torah: study is a kind of worship that you practice every day. I think of Gotthold Lessing (who, so far as I can tell, was not Jewish):
[N]ot by the possession but by the pursuit of truth are [a man's] powers expanded, wherein alone his ever-growing perfection consists. Possession makes us easy, indolent proud.[I see I quoted this earlier: link.] Yiddish certainly is a vehicle for this culture, even if it is not its creator (--Why do you people always answer a question with a question? --Do we?).
There is no doubt some point to be found in the sketch I was offering the other day--of Yiddish as one competitive voice in a noisy and fractious set of triplets, i.e., along with Hebrew and Aramaic. I could extend the point--Yiddish in its heyday was not merely part of a trilingual culture at home: it was virtually always working in confrontation with neighbor-languages: German of course, but also Russian or Polish or Ukranian--later English.
On a slightly different note, Harshav also offers insight into the process whereby Yiddish (well: a small group of talented Yiddish writers) promotes itself from the status of folk tongue to the role of a full-blown literary language. Many have remarked on how Yiddish as it grew was not a "court language"--no central bureaucracy to set standards or make roles. Harshav points out that also it was not a "bourgeois language" and so was not in a position merely to imitate the novel-writing tradition of the West. It was the Yiddish writers--Sholem Aleichem is the name best known to Americans--who figured out a way to define and occupy a "cultural space" in which the language could operate.
In the end, I suspect it was this very flexibility that killed Yiddish. A language that could come to terms with Biblical Hebrew and Manhattan English at last could live without itself at all. I think it is Wallace Stevens who said that all French words are part of English; only some we put in italics. Harshav offers a telling example, although I am not sure he sees its full importance: is Philip Roth (or Saul Bellow) a Jewish writer working in America, or an American working with Jewish themes? In the end, it tells you something about Yiddish to say that the distinction doesn't matter all that much.