E-books and electronic journal articles can contain a number which identifies them, called a DOI (Digital Object Identifier). A DOI is a string of numbers, letters and symbols used to permanently identify an article or document and link to it on the web. A DOI will help your reader easily locate a document from your citation. It is like a PPS number for the article you’re citing — it will always refer to that article, and only that one (source: UIC).
A DOI is not the same as an ISBN, but it does the same thing - it is a unique number assigned to a piece of work that allows you to find the work even if the work moves databases or websites, etc. The DOI is a hyperlink which can take you to the article in one click.
If you know the DOI it is best to include it in your references/bibliography so that you know you're linking directly to the correct article, and to make it easy for the reader to check the source.
Reference: Authors Last name, Initials. (Year). Title of work [Reader name version]. Retrieved from URL/DOI.
Example: Luhr, W. (2004). The Coen brothers’ Fargo: Cambridge University Press film handbooks series [Kindle version]. Retrieved from http://www.amazon.com/Kindle-eBooks.
Reference: Authors’ Last name, Initials. (Year) Title of book. Available at: URL (Downloaded: Day Month Year).
Example: Luhr, W. (2004) The Coen brothers' Fargo. Cambridge University Press film handbooks series. Available at: http://www.amazon.co.uk/Coen-Brothers-Fargo-Cambridge-Handbooks ebook/dp/B001G60IQI/ref=kinw_dp_ke (Downloaded: 24 February 2014).
There are two styles of MHRA referencing - footnotes and bibliography style and author-date style. Please check with your lecturer/tutor which one you must use.