In all academic fields, the best resources for students to cite are peer-reviewed scholarship. The way to locate peer-reviewed scholarship is to search indices that you can find on the website of your university library. You should starting by pointing your browser at your university library portal. Search for a resource called MLA (short for Modern Language Association) International Bibliography. Enter Mary Shelley and Frankenstein as search terms to get a list of a few hundred articles on the general topic. Click on links to articles with titles that seem relevant to your research, to read the full texts of the articles.