Slavery and enslavement in SF

Explorations of literal slavery, either as practiced historically on Earth, or other types of slavery.

Thoughtful, serious examinations of slavery

Burdekin, Katharine. Swastika Night (in a far-future Nazi regime, women are literally enslaved and used as breeding chattel)
Butler, Octavia. Kindred (1979) (classic; woman travels back in time to her African-American slave ancestor)
Others of Butler's works deal with the questions of freedom and enslavement. For instance, the Xenogenesis trilogy.
Charnas, Suzy McKee. Walk to the End of the World (in a post-apocalyptic North America, women are literally enslaved; interesting treatment because of the discussion of how people become enslaved, and how slaves work together)
Cherryh, C. J. Cyteen [Artificially designed people, or "azi's," are not technically slaves; they are considered more a sort of industrial property or intellectual property. But their legal status is clearly that of property.]
Emshwiller, Carol. The Mount (2002)
Kritzer, Naomi. Freedom's Gate (2004) and sequels.
Le Guin, Ursula K. Four Ways to Forgiveness (four stories about a world in which one race enslaves another; serious & well-written exploration of slavery)
Lynn, Elizabeth. The Sardonyx Net (1981)

(slavery used as a plot device but not necessarily examined culturally; not necessarily trashy)

Jacqueline Carey. Kushiel's Dart (2001) [people are in bond until they earn their "mark"; more "indentured servitude", time-limited slavery]
Jo Clayton. Irsud
Lee Crittenden. "Anthem" in Sexcrime: An Anthology of Subversive Erotica edited by Cecilia Tan (Circlet Press: 2000) (slave clones discover sororal sapphic sex)
Phyllis Gotlieb. Flesh and Gold
Robert A. Heinlein Citizen of the Galaxy
Frances Lucas. Cathy IV (1992). (A naive young lesbian is shipwrecked on another planet, and becomes embroiled in their slave economy, when she falls in love with a slave/robot.)
Matthews, Susan. The Jurisdiction Universe series featuring Andrej Kosciusko; the universe is full of "bond-servants", a punitive version of indentured servitude.
John Norman. Gor series. (Distinctly & specifically anti-feminist fantasy in which women are sex-slaves to men.)
Severna Park. Speaking Dreams (Firebrand, 1992) (good writing - lesbians in a universe with slaves fighting the odds to be together)
--. Hand of Prophecy (set in same universe as Speaking Dreams)