My first observation about this book is that the title (in my opinion at least) is not accurate. On seeing it I was hoping the book would be about building OO systems on top of relational databases, or on how to put an OO layer on top of them to make them look like OO databases.
In fact, the book is a comprehensive introduction to both relational and OO database technology but does not go into great technical depth. There is some material on objectifying relational databases, but that's only part of it. The book begins with tutorials on SQL and object orientation. It then moves on to relational basics and to an introduction to object databases. Then it goes on to talk about Java, the web and distributed object technologies such as COM and CORBA. There's a lot of overview but it does not go into great depth.
Now for the book's biggest drawback: although I obtained my review copy in 1998, the copyright date is 1996. I think this is quite a serious problem, given how fast things move on in fields like the web and Java.
In conclusion, the book is very easy to read and provides a reasonable introduction to both relational and object databases. If that's what you're looking for then this book may well meet your needs.