Firstly, can I say that I enjoyed reading this book. The concept is fascinating; the first half explores what actually happened up to the end of August 1940, the second part explores one possible scenario for what might have happened had an invasion actually been attempted.
I will deal with each separately. Part one gives a fascinating account of preparation for Sealion on both sides of the channel. I did however sometimes wish the author had put some of the reproduced documents and proposed orders of battle in appendices rather than in the body of the text. In places, failure to do this has led to some very dry sections of the book.
Part two of the book gives a very interesting (fictional) account of what would have happened if the Sea lion had gone ahead. It starts very promisingly and gives a very clever account of landings. Whilst I agree with the author's conclusions about the ultimate outcome of such an undertaking, parts of his narrative seem a little unconvincing. In particular, several times the scenario relies on german troops becoming demoralised, not pressing home attacks and being beaten in the field a little too easily to be entirely plausible. This runs counter to many reports of the actual behaviour of elite german units and of the morale of german units fresh from their victory in France. The book also seems to downplay the role of the Royal Navy in disrupting the resupply of any invasion force.
It would have been nice to have expanded this section of the book to include a discussion of the political situation in Britain during landings. A greater exploration of the Battle of England as the author calls it would have been appreciated. I also feel the the author has missed a trick in that there is no discussion of how a failed invasion would have impacted on the course of the war.
In conclusion, part one is an interesting read whilst part two ultimately left me a wanting more detail.