Short answer, you wouldn't.
You are referencing the book that had the information in it, not the platform you've used to read the book.
The confusion appears to lie in what you are deeming an online resource. A book or journal is its own resource, regardless of what platform or media you read it in. If referencing a book, it doesn't matter if you bought it direct, used a library or read it on a e-reader, all the relevant information is included.
This is a useful guide put out by the University of Sydney that provides multiple examples of how to reference based on multiple source types.
Online resources refer to articles/blogs that are published online only, and not a book that you have read in an online format.