Start with a keyword search in the catalog. Once you have located a relevant item, you can look at the Subjects in the record and follow those links to find related books.
You can also refine your results by using the facets on the left side of the screen.
Immigrants -- [Country or Region] example: Immigrants -- United States
[Country of origin or destination] -- Emigration and immigration example: Italy -- Emigration and immigration
(Political) Refugees -- [Country or Region] example: Political refugees -- Cuba