According to the "Publication Manual of the American Psychological Association" (6th edition), writers must properly indicate the source of words and ideas they use in their papers and publications. Information about those sources should include as much detail as possible in order to help your reader find the reference. Indicating that a book is part of a series can help with that search.
The in-text citation in APA style gives the author's last name and the year of publication with a comma between. For example: (Leon, 2012). You may also place the author in a signal phrase followed by the date to emphasize your source: According to Leon (2012). Direct quotes also need the page number with a comma before and a "p." to indicate the number is a page. For example: (Leon, 2012, p. 33).
The entry on the references page gives your reader the information needed to find your sources. To indicate which book in a series you cited, give enough information to differentiate that book from others in the series. For instance, you might include the volume number after the title. The rest of the information is just like any book entry. Start with author's last name, a comma and first initial. The date appears next, in parentheses, with a period after. Next type the title, italicized, capitalizing the first word and proper nouns, but before the next period, you might put "Vol." and the number in parentheses. Then comes the city of publication, a colon, and the company with a period at the end. Such an entry might look like this:
Leon, S. (2012). The title of the work (italicized) (Vol. 3). New York: Penguin.
If the titles in the series differ, you need no information in the entry other than the usual book format.