Walter Block is a libertarian Austrian economist. His website, which collects all his writings, is here. Below are some of his writings on the subject of immigration and open borders.

• A Libertarian Case For Free Immigration (PDF, 20 pages), part of a special issue on immigration by the Journal of Libertarian Studies.
• Is There a Right To Immigration? A Libertarian Perspective (PDF), co-authored with Gene Callahan.
• On Immigration: Reply to Hoppe (PDF, 18 pages) in the Journal of Libertarian Studies. This is mentioned on the anarcho-capitalist counterfactual page.