Chicago: University of Chicago Press, 2010. Chapter 14.2 explains the notes and bibliography system. Chapter 14.19 and following sections provide guidance on formatting footnotes and bibliography entries for books, articles, online resources, and other sources.
Completely searchable online version of this important style manual, providing quick answers to style, editing, and citation format questions (for print and electronic sources).
Purdue's OWL website features Changes in the 7th Edition, a guide to major differences APA introduced in fall, 2019.