To view an ebook while online, click the View Online button in the catalog:
A page with complete information about the ebook will open in a new tab or window. On the left side is a button for PDF Full Text, highlighted below. Click it to open and read the book online.
You may also see an option for EPUB Full Text. That's fine. They both work the same way in the online ebook reader.
When the ebook opens in your browser, you can navigate through it in a variety of ways, highlighted in the image below.