I consider myself liberal, progressive. I like writers along the same vein. Wilde, Voltaire, Thoreau. Other suggestions from known or lesser known writers and thinkers?
Henry Miller and James Baldwin are the first two writers who come to mind: both "self taught" writers; both liberal and sometimes political. Miller has a greater tendency to be philosophical, between the two, but Baldwin is very thought provoking too.
These writers are much more contemporary than those on your list though...
Two others that come to mind are Moliere and Orwell. In Tartuffe, Moliere's attacks on religious hypocrisy and the strict social order certainly mirror progressive thinking for his time. Orwell, it seems to me, would be considered progressive in his defense of individual freedom when it is threatened or crushed by the state and in his assessment of the politics of his day that led him to anticipate the growing power of the state over the individual. 1984 champions the individual and certainly sounds the alarm regarding oppression through political power. Also, in "Shooting an Elephant," Orwell explains exactly how and why he came to view colonial imperialism as an inherently evil political system.
How about Charles Dickens and John Steinbeck? Both used their stories and their position to raise awareness for the plight of the poor. Their fight for social justice did make an impact in their societies, and that impact continues to the present day.
From the Science-Fiction side of things, try Loid McMaster Bujold. She's been writing since the 1980s and many of her books involve the difference between "old" and "new" thinking. In the same vein, some of Robert A. Heinlein's later novels have strains of liberal progressivism in them, although he tended towards the libertarian side of things. You can't really pin him down, though; his books are all over the place.