The best way to find an ebook is to search in the catalog. Search for the title or topic you're interested in, and if an ebook is available it will be displayed along with the print books. On the right side of the page under "Availability," you'll see an option to narrow your results to just items that are available online.
If you click on the title of the book, you'll see the full item record. In the "View Online" section there will be a hyperlink that will take you to the ebook.