We have a several e-book vendors.
Controlled vocabulary legislate the use of predefined, authorised terms that have been preselected by the designer of the vocabulary, in contrast to natural language vocabularies, where there is no restriction on the vocabulary.
Subject headings are a controlled vocabulary. A single term or phrase is determined by an authority (Library Congress or a database vendor) to describe a subject. For example, try searching for the subject "roaring twenties" in West-Cat and see what you get.
If you are looking for topics, you might try these subject headings for monographs:
Library of Congress Subject Headings (LCSH)
For this assignment, go to advanced search and change the drop down menus on the right side to "subject words." Use the name and city, such as "Detroit" in one box, and "history" in the other. If need be, add another term, such as "race relations." Or add the heading "sources" for primary sources.
Consider using Google Books to find the full text of books published years ago.