MLA offers some citing FAQ online, including how to cite eBooks:
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.
For books that you find from the library catalog and read online in your browser, the best description seems to be Digital file, but if you download the book to read it, it would be PDF file.