Common is the universal language that is spoken throughout Elanthia. There are no Common language books, because Common is always expressed in English. Mechanically speaking, all PCs can speak Common, which is the default language. However, some people choose to role-play characters that speak only their racial languages.
The term "English" is OOC, because real-world languages don't exist in Elanthia. Instead, people use "Common." For example, people might say that the Common translation of "roisan" is "minute."