A. John Simmons recent work Justification and Legitimacy is an excellent introduction to his thought. It both clarifies and extends many of his views, particularly his arguments against the idea of political obligation, in an admirably lucid fashion. This is a fine book, well written and well reasoned. Philosophy should always be that, and Simmons does not disappoint. His criticisms have bite even if they are at times a bit misguided.
His criticisms of Ronald Dworkin are particularly irritating. He criticizes Dworkin for developing a theory which excludes the anarchist position on political obligation, while at the same time being unwilling to meet Dworkin's argument on it's own interpretivist ground. In other words, Simmons does precisely the same thing he accuses Dworkin of doing. What's worse Simmons does this with little argument on his part.
In itself, this is not a strong objection to Simmons book. It does, however, suggest that there are basic problems with Simmons's approach, namely, you end up having to accept his argument without understanding the type of argument that Dworkin's making. Regardless, this is a good book which makes a valuable contribution to political thought.