This is definitely a book to have on the bedside table and to dip into from time to time rather than something to read from beginning to end. The combination of language instruction, cultural etymology and humour sustain the interest.
Rosten's claim that Yiddish is a language which is better than any other at expressing the emotions must remain suspect. Is it really the case that Swedes or Greeks are less able to bend and inflect their languages to express what they really feel? Nonetheless, it is possible that Rosten is right in arguing that Yiddish has a disproportionally large vocabulary to express types of human behaviour.
Rosten probably also makes the mistake of exaggerating the extent to which Yiddish has penetrated the English language, or British English at least. Words like kosher, glitch, schmuck may have made it, but the vast majority are not even understandable. (Some, though, are identical or similar to German and can be understood that way)
Before Zamenhof published Esperanto in 1887, he had worked out a simplified version of Yiddish written in the Latin alphabet which he intended to be used as a lingua franca. Had that language taken off, had the Holocaust not happened and had Israel not adopted modern Hebrew as its state language, the role Yiddish plays today might be so much greater.