Q. How to I cite the introduction to an ebook?

On our Cite MLA Style (link below) there are many tools to help with your citation.  

From the Cite the Easy Way (link below) page, there are some citation generators that will assist you.  

Zoterobib is a great one and it is linked below.

Select the type of material you need to cite.  In this case, select 'Book', then the page EBook: online.

Show what you are citing (Whole Book, Chapter or Section, or Preface or Introduction.)

Fill in the information needed, the title of the section and author of that section, if needed.  The title of book, and other information about the book, then the information about the website on which you found the book.