Sourcery

Sourcery is the fifth Discworld novel by Terry Pratchett, published in 1988. The title is a pun on "sorcery"--on the Discworld, a sourcerer is a special type of wizard who is a source of magic. The first few pages of the novel deal with the sourcerer's father, who cheats Death by making a prophecy that Death must honor or risk destroying the Discworld. The rest of the novel deals with the sourcerer's plan to have wizards rule the Discworld, and the efforts of a small group to thwart those plans.

Terry Pratchett has said that he considers this the book where his writing style truly began to mature.