Generally speaking, an excellent starting point for finding primary and secondary materials relating to immigration law is WestlawNext's "Immigration" practice center.  You can use the search box at the top of the page to search all immigration-related materials, or you can select a particular type of resource from the menus on the right-hand side of the page.

Lexis provides many excellent primary and secondary materials relating to immigration law.  To access these resources, from the top of the Lexis home page, select Practice Area > Immigration. Alternatively, here is a direct link to the Immigration Practice Area page.

Lexis has "Topic Summary Reports," which typically include definitions, citations to seminal cases, statutes, and regulations, links to secondary sources, and discussions of burdens of proof and standards of review.  To access a summary report for an immigration-related topic, from the Lexis home page, select Secondary Sources  > Legal Topic Summaries > Immigration then run a search with a few keywords. Alternatively, here is a direct link to the Immigration Legal Topic Summaries search page.