You can read below what’s the aim of this book, in short it’s with not surprise a book about the 16 Daedra princes. This is again a production from that RP session, I’m not telling it’s entirely canon but I think it’s still a vision matching the lore.
You probably already know the wondrous Book of Daedra describing the spheres of the Daedra princes and maybe you think this present book is a rip off. Not exactly I would say. This book is also about the Daedra princes and their spheres but the focus is more on the “why” than on the “who and what”.
To begin I will remind you first what Daedric planes are: Daedra princes themselves. There is no real difference between the prince and the realm, neither between the prince and any lesser Daedra. They are all a same energetic entity divided in different aspects, what we call the prince is the concept governing this energy. So when you conjure a Daedroth you’re actually inviting a little piece of Molag Bal on Tamriel.