(Source:  http://blog.apastyle.org/apastyle/2011/06/how-do-you-cite-an-e-book.html)

E-books come in a variety of formats (e.g., Kindle, Adobe Digital Editions, EPub, HTML, and more) and can be read on a variety of devices (e.g., e-readers like the Kindle, Nook, and Sony Reader, as well as on personal computers and mobile devices through online portals such as NetLibrary, ebrary, and Google Books).

Reference List Entries

The reference list entry for a whole e-book should include elements of author, date, title (with e-reader book type in square brackets if applicable; italicize the title but not the bracketed material), and source (URL or DOI):

• If the book was read or acquired through an online library (e.g., Google Books, ebrary, NetLibrary) and not on an e-reader device, omit the bracketed information from the reference.

The reference list entry for a chapter in an edited e-book should be written as follows:

• If the e-book chapter does not have page numbers, omit that part of the reference.
• To determine whether you need to cite the whole book or just a chapter, please see this post. 

In-Text Citations

For in-text citations of paraphrased material, provide the author and date, as for any APA Style reference. To cite a direct quotation, also provide page numbers if the e-book has page numbers. If there are no page numbers, you can include any of the following in the text to cite the quotation (see section 6.05 of the Publication Manual, pp. 171–172):

• a paragraph number, if provided; alternatively, you can count paragraphs down from the beginning of the document; 
• an overarching heading plus a paragraph number within that section; or 
• an abbreviated heading (or the first few words of the heading) in quotation marks, in cases in which the heading is too unwieldy to cite in full.

A Note on Kindle Page Numbers and Location Numbers 

As of March 2011, many Kindle books now have real page numbers that correspond to those in print editions (as far as we know, this applies only for Kindle third generation products and going forward). These real page numbers are appropriate to use in academic citation (as are the page numbers of other paginated e-books). Kindle "location numbers," however, should not be used in citations because they have limited retrievability. Instead, for any e-book without page numbers, APA recommends the method described above for citations of directly quoted material.

See these links for further discussion of Kindles, e-books and e-book chapters, and citing unpaginated material, and see Publication Manual section 7.02 (pp. 202–205) for more examples.

(Source: 14.166 Books downloaded from a library or bookseller http://www.chicagomanualofstyle.org/16/ch14/ch14_sec166.html)

The majority of electronically published books offered for download from a library or bookseller will have a printed counterpart. Because of the potential for differences, however, authors must indicate that they have consulted a format other than print. This indication should be the last part of a full citation that follows the recommendations for citing printed books as detailed throughout this section. See also 14.4–13.

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.

Austen, Jane. Pride and Prejudice. New York: Penguin Classics, 2008. Microsoft Reader        
       e-book.

Austen, Jane. Pride and Prejudice. New York: Penguin Classics, 2008. Palm e-book.

The printed counterpart to the Penguin Classics e-book offerings would be cited as follows (note the different publication date):

Austen, Jane. Pride and Prejudice. New York: Penguin Classics, 2003.

Each of the Penguin Classics editions (as the books’ documentation makes clear) is based on the 1813 edition published by T. Egerton. Though such information is optional, it may be included as follows (see 14.119):

Austen, Jane. Pride and Prejudice. London: T. Egerton, 1813. Reprint, New York:     
     Penguin Classics, 2008. PDF e-book.

Note that electronic formats do not always carry stable page numbers (e.g., pagination may depend on text size), a factor that potentially limits their suitability as sources. In lieu of a page number, include an indication of chapter or section or other locator. See also 14.17.

1. Jane Austen, Pride and Prejudice (New York: Penguin Classics, 2008), Microsoft
         Reader e-book, chap. 23.

14. Austen, Pride and Prejudice, chap. 24.

(Source: http://www.mla.org/style/handbook_faq/cite_an_ebook)

In general, a work formatted for reading on an electronic device like Kindle, Nook, and iPad is covered by 5.7.18. Begin the entry in the works-cited list like the entry for a comparable printed work and end it with a designation of the medium of publication. The medium is the type of electronic file, such as Kindle file, Nook file, EPUB file, or PDF file. If you cannot identify the file type, use Digital file. For example:

Rowley, Hazel. Franklin and Eleanor: An Extraordinary Marriage. New York: Farrar, 2010. Kindle file.

If the work presents electronic and print publication information, the electronic information should usually be cited.

Most electronic readers include a numbering system that tells users their location in the work. Do not cite this numbering, because it may not appear consistently to other users. If the work is divided into stable numbered sections like chapters, the numbers of those sections may be cited, with a label identifying the nature of the number (6.4.2):

According to Hazel Rowley, Franklin and Eleanor Roosevelt began their honeymoon with a week’s stay at Hyde Park (ch. 2).

or

Franklin and Eleanor Roosevelt began their honeymoon with a week’s stay at Hyde Park (Rowley, ch. 2).

(The abbreviation ch. is shown in 7.4. There is a comma in a parenthetical citation after the author’s name if the following reference begins with a word.)

If the work is a PDF file with fixed pages, cite the page numbers. If the work lacks any kind of stable section numbering, the work has to be cited as a whole (6.4.1).