The Sendaikujihongi is an ancient collection of Shinto myths, and most people have never heard of it. This is even true in Japan. This is because, for about a thousand years, people believed that it had been written by Shōtoku Taishi in the early seventh century, until an astute scholar in the Edo period pointed out that it referred to events in the ninth century. After that, it was regarded as a forgery, and largely ignored.
As a result, it has been much less available — it was published, in the original classical Japanese, in the really expensive series of ancient documents, and that was about it. However, a few years ago a complete translation into modern Japanese was published, and I have just finished reading it. (There is also an English translation, but that is really expensive.)
I think the historical attitude to the text is partially justified. It is less interesting than the Kojiki and Nihonshoki, in large part because much of its content is clearly derived from them. It also contains a whole section on the genealogy of the important Mononobë clan, which is little more than a list of names, and the final book lists the “Kuni no Miyatsuko”, and the kami from which they were descended, for the ancient provinces of Japan. This includes some provinces that were created in the eighth or ninth centuries, and some provinces that were merged with other provinces before that. It’s the sort of thing that would be very boring if you were not a real Shinto nerd.
Obviously, I found it fascinating.
The translation comes with an introduction and notes. Unfortunately, they are both written by non-mainstream scholars, who believe that the Yamatai mentioned in Chinese records was definitely in northern Kyushu, and that all the figures in the myths, including Amaterasu Ōmikami, were historical individuals in Japan. This is not an absurd position (it is more respectable than believing in King Arthur, for example), but it is a long way outside the mainstream, which means that I have to use caution when looking at the notes.
The final section does mention an event from the early ninth century, so the current form of the book is no earlier than that. However, the text also uses much older ways of writing certain important place and family names (inconsistently), which suggests that it drew on earlier sources. It includes additional myths related to the Mononobë and Nigihayahi, their ancestral kami, as well as the genealogy, which strongly suggests a link to the Mononobë.
Further, it is clear that the compiler had access to both the Kojiki and Nihonshoki, because there are mythic elements taken from both texts. The clearest example is in the legends of Amë-no-Uzumë. In the Kojiki, she famously strips while dancing in front of the cave of heaven, but that is not mentioned in the Nihonshoki. On the other hand, in the Nihonshoki she exposes her breasts before confronting Sarutahiko during the descent from the heavens, but that is not mentioned in the Kojiki. The Sendaikujihongi has her getting undressed on both occasions, so we can conclude that it was compiled by someone with access to both texts. Given this, and the information and legends that are not found in either the Kojiki or Nihonshoki, it seems likely that the Sendaikujihongi was compiled by someone with access to a wide range of early texts on Japanese myth and history. When did that happen?
The Sendaikujihongi is mentioned in texts that reliably date to the early tenth century, so it is no later than that, and there is (apparently) no good internal reason to date it much later than the early ninth century, at least if we exclude the preface, which appears to have been added later. That makes it roughly contemporary with the Kogoshūi, which is treated as an important source. Overall, then, describing the Sendaikujihongi as a forgery seems unfair. The claim in the preface that it was written in the early seventh century is false, but the book appears to be a genuinely early compilation of myths, almost certainly by a member of the Mononobë.
Thus, I do plan to look at the Sendaikujihongi when doing future essays on myths, but it is not going to be the main source for very much.