The book has left out many 'Pagan' religions but, according to Jason Pitzl-Waters:
So if god is not one, how many gods are there? Prothero’s polytheism doesn’t go that route. He instead explores eight different “great” world religions (Christianity, Islam, Judaism, Buddhism, Taoism, Yoruba, Confucianism, and Hindusim), their conceptions of god, what they see as the primary problem with the world, and how they approach solving that problem (for example, in Buddhism the problem is suffering and the solution is awakening). It’s an interesting way of approaching the subject, and I look forward to seeing how Prothero presents it.
Here's a video of the author discussing the book.