In WorldCat Discovery, remember that you can limit your searches to e-books only.
Enter your search terms, and then begin looking at the limits/filters on the left-hand side of the screen. You'll scroll down through the "Format" options, and likely will need to click on the "Show More" option
Then you can click on the "e-book" limiter
After doing this, you can also choose to keep that limit for other searches during the current browsing time.
NOTE:
I do NOT recommend using the "limit to full-text" options in databases from Ebsco (like Academic Search Premier) or ProQuest (like Sociological Abstracts). Doing so does not find all the articles which are full-text through connected databases via the FindIt button.