Use the Version element to specify that the work you are citing is an e-book—that is, a book that lacks a URL and that you use software to read on a personal device or computer.

MLA Handbook. 9th ed., e-book ed., Modern Language Association of America, 2021.

E-books may have been published in different file formats (e.g., EPUB or, for some earlier Kindle editions, MOBI), and the display of content in each format can vary. If you know that the work varies by file format for the e-book you are citing, you can list the file format as a supplemental element at the end of the entry. 

MLA Handbook. 8th ed., e-book ed., Modern Language Association of America, 2016. EPUB.

When citing an e-book in your text, avoid using device-specific numbering systems. See chapter 6 of the ninth edition of the MLA Handbook for an alternative way to identify the parts of an e-book.