For this page, I'm limiting the term "eBook" to published books of any type that are available to the general public by either being read online or download to their computer as a PDF or similar file. These types of books don't require the reader to have any additional equipment such as a Kindle.
     An eBook can be placed on the internet and function much like this website. The person reading the story flips through the pages by clicking on links. Creating such a site isn't difficult and you can make the pages as plain or as fancy as you choose. The number of pages and graphics are only limited by the amount of storage space provided by the webhost provider. Bandwidth, or the number of pages that can be viewed during a given period of time (a simplified explanation) can limit the number of people who read the book during that time.
     It is possible to charge people to view your online book. Although I don't know the technical aspects, you can password protect the site and set it up that once a person pays, they receive a password. You can get paid through a service such as PayPal. The same applies to the downloadable PDF or other file.
     An alternative is to create a link to a PDF or similar file which the person can download to their computer. This method has it's advantages to both the author and the customer. The author only needs enough storage space at their webhost to create a few simple web pages where they can post a description of the book, a link to the file, and the file itself. The person who wants to read the book can open the file at any time to read the book even if they experience problems with their internet connection.
     As with many other parts of the internet, these two ways of publishing your book have a downside. No matter how well your password protection works, there will always be people who know or figure out how to get to your online book without the password. In the case of downloadable files, you lose control of the file once it's on someone else's computer. If they're dishonest, they'll pass the file along to anyone who wants to read the book so other people could be reading it for free.
     There are many free books online and offered as downloads. It's your choice whether or not you want to be paid. If you choose to be paid, hire a professional or find a friend who can help you with the technical stuff. They may be able to encrypt your files so they can't be passed off to other people.
     Important: Always post a copyright notice at your website or within the downloadable file. Tell people you own the work, control it's use, and how to contact you if they want to request permission to use any of the information.