In my software development career, I've had to use numerous Relational Database Systems, including Oracle, Sybase, SQL Server and, for my personal projects, PostgreSQL. Recently I got involved in a project that requires the use of MySQL.
I decided to get a book to get up to speed on MySQL idiosyncrasies. Browsing through Amazon's selection and reading the reviews for various of the MySQL books there, I decided to get MySQL, by Paul DuBois.
When I started reading the book, I realized that I was already familiar with most of the material presented in the book, since the book is aimed primarily at people with little or no experience in databases. I believe that this book accomplishes teaching relational database concepts to beginners, but for people, like me, who are already familiar with relational databases and SQL, it is hard to find relevant information. To be fair to the author, I did learn some things that I didn't know about MySQL from the book (for example, I was unaware that newer versions of MySQL support subqueries and foreign keys, when using the InnoDB engine), however this information was buried among a lot of information of little relevance to me.
In conclusion, if you are new to relational databases and you need to work with MySQL, this book is recommended, if you have experience with other RDBMS's, this book might not be for you.