A legal secondary source (aka secondary authority) is writing about the law (aka primary authority). Legal secondary sources have two main purposes:
Kurzban's Immigration Law Sourcebook (Ira J. Kurzban, 17th ed.) is "the bible" of immigration law which includes discussion of important cases, regulations, statutes and agency rulings. It is designed for use by practitioners and students.
It is not available online, but is available in print in the library in the reserve collection behind the circulation desk (big green desk) in the library.
There are many immigration treatises and practice guides available. You can find them by looking on Westlaw, Lexis, legal research guides, and the UA library catalog.
Here are some examples of useful sources:
Immigration Law and Procedure (Charles Gordon, Stanley Mailman, and Stephen Yale-Loehr) (available on Lexis)
Immigration Law & Defense (Immigration Project of the National Lawyers Guild) (Available on Westlaw)
Immigration Pleading and Practice Manual (Thomas Hutchins) (Available on Westlaw)
Immigration Law Service 2d (Shane Dizon and Pooja Dadhania) (Available on Westlaw)
As a reminder, the purpose of immigration law research is to find the all the relevant primary law on your topic in your specific jurisdiction: federal statutes, federal regulations, federal cases, federal agency decisions, and, if relevant, federal guidance documents, agency directives, presidential directives, executive orders, attorney general directives, state or federal statutes, treaties, foreign law, etc.
Then you can use the primary sources to analyze your issue to explain it to your client, write your memo/motion/brief, etc.
In case you think that conducting immigration law is kinda hard, you might be happy to note that Charles Gordon, Stanley Mailman, Stephen Yale-Loehr, and Ronald Y. Wada, authors of Immigration Law and Procedure, agree with you.
On this same page of the book, I found this definition of immigration law that I really like. I think it does a good job of summarizing the jumble of immigration laws.