J. R. R. wrote a great deal of material connected to the Middle-earth mythos that was not published in his lifetime; although he had originally intended to publish The Silmarillion along with Lord of the Rings, and parts of it were in a finished state, he died in 1973 with the project undone.
Christopher had been long been part of the critical audience for J. R. R.'s fiction, first as a child listening to tales of Bilbo Baggins, and then as a teen and young adult offering much feedback on Lord of the Rings during its 15-year gestation. Christopher himself had the task of interpreting his father's sometimes self-contradictory maps of Middle-earth in order to produce the versions used in the books. Christopher re-drew the main map in the late 1970s to clarify the lettering and correct some errors and ommissions
Later the son followed in his father's footsteps, becoming a lecturer and tutor in English Language at Oxford University.
After his father died, Christopher embarked on organizing the masses of his father's notes, some of them written on odd scraps of paper a half-century earlier. Much of the material was handwritten, frequently a fair draft was written over a half-erased first draft, and names of characters routinely changed between the beginning and end of the same draft! Deciphering this was an arduous task, and perhaps only someone with personal experience of J. R. R. and the evolution of his stories could have made any sense of it; even so, Christopher has frequently admitted to having to guess at what was intended.
Nevertheless, working with Guy Gavriel Kay, he was able to publish The Silmarillion in 1977, followed by the twelve volumes of The History of Middle-earth between 1983 and 1996. Although to some the Silmarillion's style is too remote and the History's analysis too stupefyingly detailed, Tolkien fans have driven these books into multiple printings, so at least financially it was a wise move to publish them, and without Christopher's interest and energy, some of J. R. R.'s best storytelling would likely have been lost in some university archive, with only the occasional researcher to puzzle over the mess.