The second part of The History of The Lord of the Rings, an enthralling account of the writing of the Book of the Century which contains many additional scenes and includes the unpublished Epilogue in its entirety. The Treason of Isengard continues the account of the creation of The Lord of the Rings started in the earlier volume, The Return of the Shadow. It races the great expansion of the tale into new lands and peoples south and east of the Misty Mountains: the emerence 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, and the original meeting of Aragorn and Eowyn, its significance destined to be wholly transformed. The book also contains a full account of the original map which was to be the basis of the emerging geography of Middle-earth.

This is the seventh volume of The History of Middle-earth. The story reaches the beginning of The Lord of the Rings and ends at the point where the author halted in the story as the Company of the Ring, still lacking Legolas and Gimli, stood before the tomb of Balin in the Mines of Moria.

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. A chief feature of the book is a full account of the original Map, with re-drawings of successive phases, which was long the basis and accompaniment of the emerging geography of Middle-earth. An appendix to the book describes the Runic alphabets as they were at that time, with illustrations of the forms and an analysis of the Runes used in the Book of Mazarbul found beside Balin's Tomb in Moria.