Also, if you need to cite a page from eNotes (a citation is a reference to a published or unpublished source), there should be a "Cite this Page" link on almost every page that will pull up a proper MLA citation.
You don't need copyright permission if you are gleaning information which is generally diffused and already known to the public, such as the date (1348 - right?) and death toll statistics (such as the fact that one out of every four Europeans died from it). Nor do you need permission if you use someone's direct words, as long as you mention the person and the source and put it off in quotations or set off in an indented paragraph after a colon, like this:
Of particular interest is Giblin's interpretation of how catastrophic events cause the balance of power in societies to shift. The Black Death, while it did not completely discredit the authority of the Catholic Church in the Middle Ages, taught the people involved that the Church could neither explain nor remedy the tragedy by appealing to its God. The people of that time reasoned that since God is the only source of life in all of its forms, He had to be responsible for what was happening to His people. They also believed that since God did nothing to help them, He must have been using the plague to punish them for their sins. When physicians rather than priests discovered how to combat smallpox, the Church lost still more of its credibility and authority.
-from eNotes reference http://enotes.com/when.plague-qn/
You do need, however, to ask copyright permission from the editor (look at the publishing company and address at the beginning of the book) for longer and more detailed use of a reference. You can also put a number at the end of your sentence and give the reference info in a footnote.
Today you may find information concerning editorial use of an Internet source at the bottom of the first page of the site under the tag 'copyright.' Copyright laws also change from one country to another. In the United States, for example, royalties to the family must be paid up to seventy years after the death of an author whose works are under copyright. Then they are declared as being under public domain and can be reprinted without charge.
You are wise to ask this question now instead of later; authors and publishing houses are cracking down on plagiarism, and they should.