Try to search “ebooks” in a search engine…
Do you think that it will include the singular “ebook” in the results?
Nope. “ebooks” is a different noun from “ebook,” and the search engine may still treat “ebook” like a name, not a regular noun. So an “ebooks” search will be treated totaly differently from an “ebook” search. So ebook is not the singular form of ebooks.
Additional thought from David Rothman: To add to the fun, you want to try “ebook” and “ebooks” both with and without the hyphen.