Joseph H. Carens, Aliens and Citizens: The Case for Open Borders, 49 The Review of Politics 251 (1987) (suggested by Patrick S. O'Donnell; on the more philosophical side, anyone wanting to understand basic normative issues in the area must read this. It's highly reprinted, available, among other places, in Will Kymlicka, The Rights of Minority Cultures (1995) suggested by Matt).
Joseph H. Carens, Migration and Morality: A Liberal Egalitarian Perspectivein Free Movement: Ethical Issues in the Transnational Migration of People and of Money (Brian Barry & Robert E. Goodin, eds.1992) (suggested by Patrick S. O'Donnell).
Howard F. Chang, Liberalized Immigration as Free Trade: Economic Welfare and the Optimal Immigration Policy, 145 U. Pa. L. Rev. (1997) (now slightly old, but an excellent over-view of the economic argument for liberalized immigration; suggested by Matt).
Howard F. Chang, Liberal Ideals and Political Feasibility: Guest-Worker Programs as Second-Best Policies, 27 N.C. J. Int'l L. & Com. Reg. 465 (2002) (for a much more clear-headed discussion of guest-worker programs than one normally sees; suggested by Matt).
Joseph Heath, Immigration and the Social Contract, 10 Can. J. L. & Juris. 343 (1997) (two best philosophical articles on immigration; suggested by Matt).
Karen Musalo, Irreconcilable Difference? Divorcing Refugee Protection from Human Rights Norms, 15 Mich J. Int. Law 1179 (1994) (gives a strong argument for generous readings to refugee protection provisions; suggested by Matt).
Stephen Perry, Immigration, Justice, and Culture, in Justice in Immigration (Warren F. Schwartz, ed., 1995) (two best philosophical articles on immigration; suggested by Matt).
Immigration and Naturalization Law Books
Free Movement: Ethical Issues in the Transnational Migration of People and of Money (Brian Barry & Robert E. Goodin, eds., 1992) (suggested by Patrick S. O'Donnell) (The Barry and Goodin volume Patrick mentions has some good stuff in it but I didn't mention it because the articles are quite uneaven to my mind and Carens' article in that volume is mostly a shorter version of "Aliens and Citizens". Still, it's a worth-while anthology for anyone interested in the area and, unlike the Scwartz volume I mentioned you can almost always find a reasonably priced copy of Barry and Goodin on amazon while the other is a part of the terribly priced Cambridge Studies in Philosophy and Law (great series but only a few volumes for less than $90.); suggested by Matt).
Guy Goodwin-Gill, The Refugee in International Law (3rd ed. 2007) (for the refugee law aspect of Immigration law. It's careful, detailed, and through and also fairly enjoyable to read; suggested by Matt).
Michael Walzer, Spheres of Justice: A Defense of Pluralism and Equality (1984) (also necessary for understanding the normative issues might be chapter 2 of Michael Walzer's Spheres of Justice, "Membership", where Walzer defends the rights of states to limit immigration on cultural grounds; suggested by Matt).