The Treason of Isengard is the seventh volume in Christopher Tolkien's History of Middle-earth and the second in his account of the evolution of The Lord of the Rings. In this book, following the long halt in the darkness of the Mines of Moria with which The Return of the Shadow ended, is traced the great expansion of the tale into new lands and new peoples south and east of the Misty Mountains; the emergence of Lothlorien, of Ents, of the Riders of Rohan, and of Saruman the White in the fortress of Isengard.

In brief outlines and pencilled drafts dashed down on scraps of paper are seen the first entry of Galadriel, the earliest ideas of the history of Gondor, the original meeting of Aragorn and Eowyn, its significance destined to be wholly transformed. Conceptions of what lay ahead are seen dissolving as the story took its own paths, as in the account of the capture of Frodo and his rescue by Sam Gmgee from Minas Morgul, written long before J.R.R. Tolkien actually came to that point in the writing of The Lord of the Rings.