Which is the constructed language which has the largest body of literature, including both works that have been originally created in that language as well as works written in natural languages which have been translated afterwards?
According to magazine author Arika Okrent in "Discouraging Words" from Failure Magazine (July 21, 2009), it's Esperanto. She writes,
What is the most widely used invented language?
Definitely Esperanto, which is ironic because when you say “Esperanto,” most people say, “Didn’t that die out in the 1920s?” Or, “Esperanto? That failed utopian project?” But in terms of invented languages, it’s the most outlandishly successful invented language ever. It has thousands of speakers—even native speakers—and that’s a major accomplishment as compared to the 900 or so other languages that have no speakers.
That's not hard to believe either. According to Esperanto.net,
Professor Sidney S. Culbert of the University of Washington, Seattle, USA, has done the most comprehensive survey on language use ever attempted. He has conducted interviews in dozens of countries around the world and tested for "professional proficiency", i.e. much more than just "hello, please, goodbye".
Based on this survey, Prof. Culbert concluded that Esperanto has about two million speakers worldwide.
Obviously, this doesn't quite answer your question, since it was specifically about published literature. However, according to this answer on the Esperanto stack, there are probably around 200 novels that were originally published in Esperanto, along with 400 volumes of short stories - and that's in the fiction category only. There's a list of Esperanto libraries on wikapedia, some of which have thousands of works in them.