This is one of the best political philosophy books that I've read in a long time. While authors like Nozick and Buchanan accept the normative ideal of a purely contractual society, but reject anarchy out of practical considerations, Mark Weiner accepts the practical viability of anarchy, but contests the value of its normative ideal. His argument is basically this:
1. A purely contractual society rests on individual property rights being defined. Without this prerequisite, there cannot be voluntary exchanges and contracts.
2. Property rights have to be enforced in order to be meaningful. (That's why, e.g., natural rights theory is non-sense.)
3. The enforcement of property rights by a state (legitimate monopoly of violence) makes it possible for everyone to have the same fundamental rights. This is possible, but not necessary, as the example of dictatorial states shows. The degree of legitimacy of a state, from a liberal point of view, is given by the extent to which everyone has the same fundamental rights, i.e. the state enforces the rights in a non-discriminatory fashion. (We can quibble about the exact list of "fundamental rights", but his argument does not depend on making this list explicit or on adopting a particular list.)
4. The enforcement of property rights by private clubs leads to a variety of rights, depending on the club to which you are a member of. In a world without government, fundamental rights (i.e. which everyone should have) no longer exist.
5. Equality of fundamental rights is an essential liberal value that should not be abandoned.
Therefore, a society of private clubs (i.e. anarchy), although it is possible, it is against the liberal individualistic ideal (i.e the ideal that everyone should have the same fundamental rights) and should be rejected.
A lot of the book is about arguing that anarchy is not only possible, but actually it has been the rule in most of human history, and it still the prevailing constitutional arrangement in much of the world. The traditional form of anarchy is not a society of private clubs, but a society of kin-based clans. His argument is that a move towards a society of private clubs, via the weakening of state power in liberal democracies, leads us back to the past in the sense that the idea of fundamental (universal and equal) rights is eroded.
His discussions of historical examples are well worth reading, for example he disagrees with David Friedman that medieval Island had been an example of anarchy. His discussion of Islam as an attempt to replace a clan-based society with a rule of law society is very interesting - he has a comparative analysis of developments in Britain and the Arab world at around the same time -, but he ultimately doesn't have an explanation for why Islam has been so much less successful than European states at eliminating the clan-based organization of society. He also doesn't talk at all about technology, but even so his historical analyses are still very good (I don't think he tries to get into the "ultimate causes" debate, he keeps it fairly descriptive).