MLA makes a distinction between online books and ebooks. Ebooks are normally downloaded onto personal devices, such as a Kindle Reader or iPad, and lack a URL.
MLA Handbook. 8th ed., e-book, Modern Language Association of America, 2016.
If you know the e-book provider, specify it:
MLA Handbook. 8th ed., Kindle ed., Modern Language Association of America,
2016.
"Ask the MLA." MLA Style Center, Modern Language Association of America, 2016,
style.mla.org/category/ask-the-mla/.