The MySQL Reference Manual is the primary source of information on MySQL. It is available online in several formats and languages from the MySQL AB Web site (http://www.mysql.com/). It is also available as a printed bound volume from MySQL Press in English and from MITP in German.
The portals section at the MySQL AB Web site provides descriptions of a number of MySQL-related books. These might be helpful in supplementing the material in the MySQL Reference Manual, and many of them also deal with specific topics not extensively covered by the manual, such as Web site development, e-commerce, and application programming using the C client library or other APIs.