I am extremely doubtful that anyone that reads about philosophy, has not read about philosophy of the law. If you've ever stumbled upon Hobbes, Locke, etc. Even Kant wrote about the law. The law, and its philosophy, are almost everywhere. The best you could do is to check an Introduction to Law or Jurisprudence course at a law school. Most of them provide such introductions. I acknowledge that they are not short, but the insighful chapters are not so long. I personally recommend the following list (which is from a Jurisprudence course taught at the University of los Andes):
There is an Oxford Handbook of Jurisprudence.
I would encourage reading these first ("introductions"):
Hans Kelsen, Pure Theory of Law (1934), pp. 193-278.
H.L.A Hart, The Concept of Law (1960), pp. 124-154 (THIS IS THE OPUS MAGNUM OF PHILOSOPHY OF THE LAW)
Duncan Kennedy, A Critique of Adjudication (1996), pp. 1-70, 157-212, 215-297 (AND THIS IS THE MOST RENOWNED CRITICAL APPROACH)
This deals with the problem of interpretation:
Ronald Dworkin, Law’s Empire (1988), pp. 1-15, 31-76, 176-258.
These are not "introductory texts", rather concrete theoretizations:
Thomas Hobbes, Leviathan (1640), chapters 5; 14-19; 26-29 (Positivism, the law is only the one emanated from the State)
John Locke, Second Treatise on Civil Government (1690), chapters 1-9 (Natural Law, there are rights and law before the existance of the State)
Savigny, The System of Modern Roman Law (1852), pp. 6-53.
Gustav Radbruch, Introducción a la Filosofía del Derecho, México, D. F., Fondo de Cultura Económica (1951), pp. 23-52, 171-180
Also not introductions, but critical approaches
Oliver Wendell Holmes, “The Path of Law” en 10 Harvard Law Review 457 (1897)
MacKinnon, C. A. (1982). (Legal Realism)
Francois Geny, Método de Interpretación y Fuentes en Derecho Privado Positivo (Comares, Granada, 2000), pp. 1-15 y 100-156.
Feminism, Marxism, method, and the state: An agenda for theory. Signs: Journal of women in culture and society, 7(3), 515-544.
MacKinnon, C. A. (1983). Feminism, Marxism, method, and the state: Toward feminist jurisprudence. Signs: Journal of women in culture and society, 8(4), 635-658.
March, James & Johan Olsen. 2006. “Elaborating the ‘New Institutionalism’”. In: Rohdes, R. A. W, Sarah Binder & Bert Rockman (eds.), The Oxford Handbook of Political Institutions. Oxford: Oxford University Press: 3-20.
Marxism on the law
Althusser, Louis. Ideology and ideological state apparatuses. In: Lenin and
Philosophy, and Other Essays. . New York: Monthly Review Press, 2001.