The appeal here, of course, was “filling a gap” in my knowledge, having that Pokemon-esqe desire to “collect 'em all” when it comes to material in my varied areas of study … in which Japanese religion was significantly under-represented. Frankly, this book provides a very handy encapsulation of Japanese history, as, obviously, the development of the intertwined religious manifestations occurs within the context of the culture, so this essentially is a 2-for-1 reading, providing data on both the history and religion.
The book starts with a general overview, and then jumps way back to the Neolithic era, which is when the first human traces appear in Japan, expressed in a hunting/gathering/fishing lifestyle, with evidence of burial rituals, and indications of typical fertility/fetishtic art. As Japanese culture evolved, so did these “native” religious elements, with one stream evolving into what was to be Shinto, with the shamanistic “Kami” spirits developing into a pantheon of deities such as the goddess Amaterasu, from whom the Imperial lineage is held to descend.
Japan, while somewhat isolated, still had a good deal of contact with its neighbors, and, in the sixth century CE, Buddhism came to it via Korea. As identified as Buddhism is (especially in the form of Zen) with Japanese culture, it's hard to think of it as a “foreign religion”, but that was what it was at the time. Various schools established themselves in Japan, and there is a long history of their competition and vacillations of influence. A century or so later, Confucianism (along with Taoist trappings) also made its way over from China, and set up the main elements that were to be in play for the next centuries.
There is a lot of details on which sects were in control, in company with which political elements, from period to period, but there's no convenient way for me to work that in here. Of course, at one point Christianity got a toe-hold in Japan, creating another axis for political influence, but this largely stopped in the years following the 1614 edict banning Christianity, expelling the missionaries, and eventually leading to bloody purges.
Now, given as my copy of Japanese Religion, Unity and Diversity is the 1969 edition, this specific volume is likely to be difficult to find ... however the on-line guys have copies of a later (1982) version for as little as one cent (plus shipping) for a “very good” copy. I rather enjoyed reading this, as it's fascinating info, stuff I hadn't known about (in detail) before, and reasonably engagingly written (for a college text). If you have an interest in how Buddhism, Shinto, Confucianism, Taoism, Zen and other offshoots developed in the Japanese cultural context, you might want to see about getting a copy from the Amazon used guys.