One great way to turn one good source into more, is to find out if a really useful author has written more on the subject. Search on the author's name, and make sure you specify that you want to search using the Author field.
Here's how you would find journal articles by the author of your WR 121 textbook in an article database:
Writing Tutorial > 5. Using One Good Source to Find More > 2. Using One Source to Find More >