This is an excellent book aspiring DBAs, developers who need to get up to speed in Oracle, IT business systems analysts and business power users.
Since this book has such a wide potential audience I'll highlight the chapters that will be of most interest to each segment. All readers will benefit from the introduction and chapter 1, which explains relational databases and entity-relationship diagrams. In particular, the E-R diagrams plus the clear discussion of keys and joins are the roadmap to the data and need to be understood by less technical users who want to fully exploit the power of SQL and PL/SQL, which are covered later in the book.
Chapters 2 and 3 cover material that more technical users will find useful: "Building the Database with the Data Definition Language" and "The Data Control and Data Manipulation Languages and the Data Dictionary". I especially liked the fact that my favorite utility, TOAD, was covered in this section - if you are a new DBA chances are you already use it. If you're a developer, chances are that you'll add it to your development environment.
The heart of this book is in Chapters 4-12. These chapters are for all readers, and they thoroughly cover SQL*Plus in detail. By the time you've finished these chapters (assuming you work through the practice exercises and put in effort) you'll be a SQL*Plus power user.
Chapter 13 covers data warehouses and shows how to use Oracle Discoverer 4.0. This material is for both technical and business users. The remaining chapters cover PL/SQL, which is aimed at the technical users. The coverage of PL/SQL is not nearly as deep as the coverage of SQL*Plus. Since most business users and the intended technical audience will benefit more from the in-depth treatment of SQL*Plus, the PL/SQL material is, in my opinion, a good introduction that can be followed up in a book, such as "Oracle9i PL/SQL Programming" by Scott Urman.