Leaving a review for this book seems kind of silly because it seems to be the only book available for a Sahidic Coptic learner, but I felt it was so good that I have to share my experience.
Many language textbooks are written in a conversational style that attempts to coax the reader into progressing by leaving a trail of crumbs of culture or history. This book is terse. Grammatical rules are numbered like theorems in a math book, and Layton is unbending in never commenting on anything that does not have to do with the rule at hand. Layton promises that if you make it to the end, "you will be ready to read the Gospel of Mark in Coptic".
Structurally, the book is simple and consistent. It is divided into 20 lessons. Every lesson has several numbered grammatical rules, followed by a vocabulary list and some exercises. Starting about half way through, selections from the New Testament are introduced to acquaint you with real Coptic. It took me about 150 hours to complete the book, which includes doing the exercises and using Anki to memorize the vocabulary.
Stylistically, I found Layton's no-nonsense tone dry at first, but I quickly came to appreciate being able to quickly find what I need without having to sift through fluff. Indeed, Layton anticipates this with his numbering system, and frequently cross-references different rules by their number. (For instance, in an exercise where a particular grammatical construction is used that the reader hasn't seen in several chapters, he helpfully leaves the rule number, knowing that the reader might need to jog their memory on it.)
My biggest complaint with the book lies in how, at two points, Layton lays an enormous heap of vocabulary (~100 forms, when the usual amount had been 15-30 per lesson) before the reader, unencouragingly writes something like "All these verbs occur more than fifty times in the New Testament and must be learned thoroughly", and carries on with the lesson. So far this might have been bearable, but what makes this frustrating is that he later introduces words from these lists in the vocabulary sections of subsequent lessons, which seems to imply that he didn't actually expect the reader to memorize those lists entirely before proceeding to the next lesson! I should have been on guard after the first time this happened in lesson 9, but I fell for it again in lesson 11. This was probably more of a problem for me than it would be for most people since I had to finish this book in 4 weeks instead of the more usual 12 (leaving more time between lessons for memorization), but still, I felt that it would have been much better for morale regardless if Layton had made his expectations about these lists clear.
This was frustrating, but this book is so fantastic in every other regard that I can't bring myself to take away a star. The exercises were largely of an appropriate difficulty, and as far as grammar is concerned, every chapter had an manageable amount of new rules introduced. If you need to learn Sahidic Coptic, I guess you don't really have a choice in whether or not to use this book, but be assured that it'll make things as easy as they can be.