The following passage sums up the entire book quite well: "[I]n...a system in which each is allowed to use his knowledge for his own purposes the concept of `social justice' is necessarily empty and meaningless, because in it nobody's will can determine the relative incomes of the different people, or prevent that they be partly dependent on accident. `Social justice' can be given a meaning only in a directed or `command' economy (such as an army) in which the individuals are ordered what to do; and any particular conception of `social justice' could be realized only in such a centrally directed system. It presupposes that people are guided by specific directions and not by rules of just individual conduct. Indeed, no system of rules of just individual conduct, and therefore no free action of the individuals, could produce results satisfying any principle of distributive justice...In a free society in which the position of the different individuals and groups is not the result of anybody's design--or could, within such a society, be altered in accordance with a generally applicable principle--the differences in reward simply cannot meaningfully be described as just or unjust." (pp. 69-70)
As with Robert Nozick (and with John Locke before them), justice is for Hayek a matter of process rather than results.
Law, Legislation, and Liberty was intended as a sequel to The Constitution of Liberty, in that Hayek wrote it to "fill in the gaps" that he felt existed in his argument in that earlier work. He wrote and published Law, Legislation, and Liberty on and off over a time-span of approximately 15 years (early-mid 1960 to mid-late 1970s), which were in part interrupted by ill health. Hayek admits that the result is at times repetitive and lacking in organization. The reason why he did not go through the effort of redoing the entire work upon completion is because he thought he might at that rate never finish it (he was 80 years old by the time volume 3 was published).
There are still plenty of great insights, which Hayek argues persuasively and in doing so manages to portray as common sense. There are also plenty of flashes of that true rhetorical brilliance characteristic of Hayek that can make his writings such a feast to the ear and mind. On the downside, however, these rhetorical gems are hidden in a large volume of pages that at times do indeed seem tedious, repetitive, and unorganized, unlike with The Constitution of Liberty, where they literally seem to jump off the page at you. All in all, read The Constitution of Liberty first, as Hayek himself suggests. And if you're not up for reading the approximately 500 pages that make up the complete Law, Legislation, and Liberty, two chapters (30 pages total) in the book The Essence of Hayek make for a comprehensive summary exposition of the ideas in the entire trilogy ("Principles of a Liberal Social Order", ch. 20 in The Essence of Hayek, covers vols. 1-2, and "Whither Democracy?", ch. 19, covers vol. 3).