Finding books on a topic is easy with subject searching. Use the Library's Browse Search. In the browse by subject option type a subject heading. Subject searches use Library of Congress subject headings and thus are always in English. For related subject terms, scroll through the list of results.
Examples of relevant subject headings for immigrant literature in Germany (both books about immigrant literature as well as examples of immigrant literature):
Immigrants' Writings, German
German literature, foreign authors
German literature, Turkish authors
Intercultural communication in literature
Multiculturalism in literature