I'd assume that just about everyone who would pick up this book are at least vaguely familiar with the current divisions between Reform, Conservative, Orthodox, and ultra-Orthodox Jews. And perhaps a bit confused about the added layer of Ashkenazi, Sephardic, and Mizrachi Jews. But maybe couldn't say where the various denominations came from, how long they've been around, and what happened in between the Talmudic sages and modern times.
The book starts in the first century, with the writings of Josephus, and great characters like Herod, Hillel, and Shammai. We learn about the Sadducees and the Pharisees, and a third contemporary group called the Essenes, of Dead Sea Scroll fame. And let's not forget the Samaritans, and the early Christians. Then the temple is sacked, the people exiled, Rome falls, the Talmud is written, the Arabs invade, and suddenly we're in medieval Europe, with Rashi and Maimonides, then on to the Kabbalists, the Enlightenment, the centuries long effort to codify the Halakha, before finally arriving at modern times.
In every age we see that there various groups with different interpretations, different approaches, and different traditions. So the current fissures between various Hasidic sects, what to do about women and gay people, and attitudes towards Israel are nothing new, just further complicated by things like the Internet. Seeing what changed over time and what stayed constant provides some insight into how the religion adapted to its surrounding, including influences from its Muslim and Christian neighbours.
I would have liked a bit more examination of not just the 'what' but also the 'how' and 'why' the Jews managed to retain their heritage, in scattered communities, usually a poor and oppressed minority. I know it was already a lot just to describe the major trends, but it just made me more curious to understand why some of the things happened the way they did. The book also largely treats Judaism as a religion, and aside from a few mentions doesn't really embrace the notion of the Jewish People as a nationality. But it's already a long book and covers a ton of material, so I really can't complain too much that it didn't focus more on my own particular interests.