The O'Reilly response is mSQL and MySQL by Randy Jay Yarger, George Reese, and Tim King. It covers both C and Perl methods to access MySQL/mSQL. Once you get your feet wet, developing a basic understanding of the DBI methods, you will likely want to know more about SQL in general. I found it quite helpful when I was getting serious about DBI.

For more documentation, you can either use the MySQL On-Line Documentation, which is quite good, or SQL In A Nutshell, also from O'Reilly, which handles MySQL, Oracle, and others. It provides much of the same information as the MySQL documentation.

The trick is not only learning DBI, but really learning how to use SQL effectively.