This book makes the list for being one of the first books I read that helped expand my understanding of the diversity and largeness of the Christian tradition. Jenkins shows how Christianity has numerically exploded in the Global South since 1900. He also shows, and goes into more depth in future books (which are also worth reading!), that the form of Christian faith in the Global South is quite different then what we may be used to. If your stereotypical Christian is a wealthy white man, Jenkins will help you see that, statistically, a stereotypical Christian is probably a poor dark-skinned woman.
Another book of his that deserves mention is his history of Christian faith in Africa and Asia, outside the usual history we learn in seminary and university. Honestly, I might have liked that one even better! At any rate, Christianity is diverse today and has always been diverse and Jenkins’ work helps show that.