You can narrow down a search in the catalog to look for only eBooks.
1. Type keywords into the search box and run the search
2. Choose "Online at My Library" and "Books/eBooks" from the menu on the left of the results page (circled in the screenshot below)
3. If you want to continue to search for only eBooks, choose "Remember all filters" from the "Active filters" section of the menu
NOTE: A search in the catalog only looks for keywords in the catalog record (author, title, etc), not in the text of the book.