Gray, John. Mill on Liberty: A Defense, 2nd ed. London: Routledge, 1996.

From the publisher:

Mill on Liberty was first published in 1983 and has become a classic of Mill commentary. The second edition reproduces the text of the first in full, and in paperback for the first time. To this, John Gray adds an extensive postscript which defends the interpretation of Mill set out in the first edition, but develops radical criticisms of the substance of Millian and other liberalism.

Online:
Higher Intellect [pdf]