There is still some confusion regarding how to cite eBook Readers.
"As e-reading devices gain popularity, professors and students are struggling to adapt them to an academic fundamental: proper citations, which other scholars can use. The trouble is that in electronic formats, there are no fixed pages." (Tushar Rae, "E-Books' Varied Formats Make Citations a Mess for Scholars: Kindle, Nook, and other devices put the same text on different pages," Chronicle of Higher Education, Feb. 6, 2011)
What is common to all citation formats is that the goal remains the same: have enough information so someone else can find it. Check with your citation guide's publications (books, websites, newsletters, blogs) to find out what's being said about e-Readers as some of them may have informally published guidelines to follow while others may offer suggestions.
When in doubt, ask your professor!
If you are the professor and you are still in doubt, then contact the publication to which you are submitting your paper. If you are just giving your students advice, and there is no information available on any of your guides' websites, RSS feeds, etc., then you can either make your own rules (it is your class!) or have your students think about what is important and necessary in order to be able to find that particular resource again.
Generally follow the format for a book, but include the format consulted at the end (for example, Kindle edition, Microsoft Reader e-book, Palm e-book, PDF e-book, etc.).
Austen, Jane. Pride and Prejudice. New York: Penguin Classics, 2007. Kindle edition.
Austen, Jane. Pride and Prejudice. New York: Penguin Classics, 2008. PDF e-book.
Optionally, you may include details about the print edition of which the digital edition is a reprint.
Austen, Jane. Pride and Prejudice. London: T. Egerton, 1813. Reprint, New York: Penguin Classics, 2008. PDF e-book.
For endnote citations, in place of page numbers, use whatever sort of chapter, section, or location numbers are provided by the e-book.
1. Jane Austen, Pride and Prejudice (New York: Penguin Classics, 2008), Microsoft Reader e-book, chap. 23.
For more details, consult the Chicago Manual of Style section 14.160. The Manual is available at the Library Service Desk on the second floor of the Library, and is also available online.
From post: "Rather than add page numbers that don’t correspond to print books, which is how page numbers have been added to e-books in the past, we’re adding real page numbers that correspond directly to a book’s print edition."