Be aware that immigration is a controversial subject. Many people are passionate about and have very strong opinions about the topic and may present information in a biased and disingenuous manner. You must assess and fact check the information you are finding and be aware that bias may be present.
Remember, even statistics can be manipulated and twisted to show a particular point of view.
What do you think about this site? Center for Immigration Studies
From their about us:
"The Center for Immigration Studies is an independent, non-partisan, non-profit research organization founded in 1985. It is the nation's only think tank devoted exclusively to research and policy analysis of the economic, social, demographic, fiscal, and other impacts of immigration on the United States.