For some reason, I keep being disappointed by this series, although it is definitely quite informative and enthusiastic about its content. You will learn a lot about the religions in question, but you probably won't come away from any one volume with a thorough understanding of how the practitioners go about their daily lives. For that, you'll have to incorporate other sources, and for me, that means, mostly, the television and movies I've seen depicting Muslim worship and society.
One thing this book put into clear perspective for me was the intrinsically divisive and fractious nature of this particular religion. Almost from the very start, it has been wracked by violent schismaticism. It's only to be expected, I suppose; any religion that claims there is only ONE way to worship God will, against a backdrop of cultural diversity, find itself divided against itself. As is the case with many religious traditions arising from the original Abrahamism, I find myself thinking that most of Islam's infighting-related problems could be resolved if its practitioners realized that Mohammed was preaching, not to the entire world, but to the people of a specific time and place.
As regards religion's hidden, sociological purpose, Islam is a mixed bag. Religion arose in order to promote group cohesion as human society became increasingly populous and diverse, during the transition from foraging / pastoralism to civilization. And even today, we see Muslim holy warriors pouring into countries that they see as under attack from anti-Islamic forces, bound to the presumed victims by religious kinship bonds. Even so, we also see entire nations regarded by other entire nations as apostate, thereby giving rise to the generational enmity between Saudi Arabia and Iran.
But I digress; this discussion goes well beyond the scope of the book. If you're interested in the world's religions, this is an indispensable volume, but as with the others in the series, don't rely on it alone to inform you of the religion's true nature.