The Treason of Isengard is the second in a series of four books, collectively know as The History of The Lord of the Rings, which is part of a larger series of 12 volumes, known as The History of Middle-earth. Christopher Tolkien gives authorship to his father, and claims only editorship for himself. This is typical of these works that he has complied and edited using his father's papers and notes, from The Silmarillion to these books. It may be true that he is editing his father's work, but he has done much more than that in these books as far as I can see. This, much like the first volume I read a few years ago, is a very detailed work of literary analysis and commentary on how the LotR was written, and how the story developed over time, as J.R.R. Tolkien wrote, edited, re-wrote, and re-wrote again, constantly outlining, drafting, brainstorming and then writing again. Every new idea he had, created more and more re-editing work to tie the new and developing storyline back into what he had already written in previous chapters.
Christopher Tolkien has undertaken an astronomical project and researching, organizing and analyzing his father's manuscripts. Most of the writing is on unnumbered, often loose-leafed sheets, scraps, envelopes, and in this book, he describes a large section written on used examination booklets, blue covers and all. Most of the writing is done every quickly in pencil, often in a kind of shorthand, with unfinished words, and then written over in pen directly over the pencil. Sometimes, he went back and erased the pencil so the original writing is lost, or he slipped in papers or strips with edits, drew pictures, editing, adding and crossing out as he went, and then went back and edited again later. Once he was satisfied, he'd then copy out a fair copy in pen in neat handwriting. Often, this copying out was actually done by Christopher Tolkien, who at one point in this book, called himself his father's amanuensis. That's something I don't remember reading before, although I do remember reading that he penned some of the maps based on his father's sketching, and he actually discussed that quite a bit in this book. As amanuensis, Christopher Tolkien is indeed in a rare position to research and interpret his father's writing, being accustomed to reading his writing, and interpreting his shorthand.
This book follows on from the first in the series, and a large part of it is actually dedicated to the completion of the Fellowship. The Two Towers is also examined, and it appears that most of that volume was included in this book, but I won't know for sure until I read the next one. But I'll probably take a break from Tolkien for a while. Its been a long ride, and this book is extremely dense.
If you're a diehard Tolkien fan, AND you're interested in the nitty-gritty of writing, then this book might be for you. Its well written, informative, and exhaustive in its depth. If you're not interested in these things, then I'd skip this one.