What to read
Citation: Thomas Hobbes. Leviathan. 1651 (Edited by Edwin Curley, Hackett Publishing Co., 1994).
Read: the first three paragraphs of chapter 15 (“Of Other Laws of Nature”) and all of chapter 17 (“Of the causes, generation, and definition of a commmonwealth”).
Where to find it: there should be multiple copies of Leviathan in just about any library. You can find just about everything Hobbes, Hume, and Locke wrote in the InteLex PastMasters series, a searchable database on the web [requires University of Chicago connection]. Look under “British philosophy 1600-1900.”
What to look for
Hobbes, like Hume, held that justice is artificial, that is, a matter of social arrangement rather than a straightforward product of human desires. Hobbes and Hume had very different reasons for thinking that justice is artificial and they gave different characterizations of the social arrangement that produces it.
Hobbes thought that justice is not natural because people naturally tend to seek to dominate one another. The social arrangement that establishes justice involves authority: the sovereign imposes and enforces rules. (I should add that his view is a little more subtle than this but the relevant qualifications are not important here).
Hume had a sunnier view of human nature and the social arrangements that produce justice, on his view, are not the product of an authority.