The library catalog only looks for the words you enter in a few places --book title, author's name, publisher, and official subjects assigned to the book. Using broad, general search terms is usually better than being too specific.
Put the word AND between search terms:
"color purple" ANDcriticism
Alice Walker ANDcriticism
"community college" ANDsuccess
"community college" ANDoutcomes
college degree ANDcost
college degree ANDvalue
Use an asterisk (*) to find variations of a word. Crit* would find anything referring to critical, criticize, criticism, etc.
To specify ebooks only in your results, select the "Full Text Online" filter.