Two books in one.
On Liberty is the work that, more than any other, establishes the basis of modern Liberalism and I can certainly see why. Some of the writing style feels (inevitably) a bit dated, but the themes and principles discussed are as relevant now as they ever were - if not more so.
The Subjection of Women was published in 1869 and argues for equality between the sexes. In many ways, this essay takes the ideas established in On Liberty and applies them to an issue of the time. This alone makes for interesting reading but it is also surprising just how many of Mill's points are still relevant today.
Published June 19, 2006 by Penguin.