I must immediately explain that very little of the technical substance of this book is new. I've said most of it before, in previous books and other publications I've just looked around and seen that it needs to be said again. But I've tried to say it differently this time: the sequence is different, the development is different, the style and treatment are different, and the intended audience is different (more on this last point in a moment). So while parts of the material have appeared before in some form or another in a variety of places, I do regard this as a totally new book. Of course, some portions of the text are, inevitably, similar to things I've written elsewhere, because the material all comes out of the same place, as it were: namely, my own brain, and my experience in teaching this material in live seminars over many years. But there's no direct plagiarism; direct plagiarism wouldn't serve my purpose. However, I have consciously reused many of my old examples, because those examples have been very carefully tailored over the years to illustrate exactly the points I want to make, no more and no less.
Let me come back to that point about the intended audience for this book being different. As already indicated, I've published several previous books in the field of database technology. So how is this one different? In particular, does it compete with any of those existing books?
In my view, the answer to the latter question is no. I have two books from Addison-Wesley that might look at first sight as if they could be competitors to this one:
However, the first of these, though I call it an "introduction," actually covers the whole of the database field, not just the relational model. It's meant primarily as a college text, and it doesn't assume any prior database knowledge or experience on the part of its readers; also, the style is much more formal than that of the present book, as befits a textbook.
The second is an extensive reworking of an earlier book by Hugh Darwen and myself called Foundation for Future Database Systems: The Third Manifesto, Second Edition (Addison-Wesley, 2000). This one is an advanced (graduate-level?) text, and it's even more formal not to say terse than the first book. Although there's obviously some overlap in subject matter, therefore, I don't really see any of these three books as competing with the other two.
Another significant point of difference is that the present book is mainly meant for self-study (though there are portions you might want to discuss with your friends and colleagues and coworkers). There are exercises, too, to help reinforce the material; there's no obligation to do those exercises, of course, but I think it's a good idea to have a go at some of them at least. Answers, often giving more information about the subject at hand, can be found online at http://oreilly.com/catalog/databaseid.
While I'm on the topic of possible competition, I should mention a couple of other books of mine (the first from Addison-Wesley again, the other from Morgan Kaufmann):
In my opinion, the first of these complements the present book, in that it reviews and analyzes, in a fairly informal style, the series of papers by Ted Codd that first introduced the relational model to the world at large. And the second is concerned, as its title indicates, not with relational theory as such but with a specific application of that theory. While the first chapter of that book does contain an overview of the relational model and thus might be considered to compete slightly with the present book, I don't really think it does.
The net of all of the above is this: although I've written on most of these topics before in a variety of places, and sometimes in unavoidably similar terms, I don't think any other publication by myself or anyone else, so far as I know brings them together and covers them in a way that's even close to the way the present book does.