Overall, it's an interesting read, and I like reading about linguistics and cultural differences so the main topic of the book is quite (meant in the American sense) spot on for me. The moments when she discusses the cultural loss of her daughter to the UK were some of my favorite moments in the book, and the peculiar faux pas that arose whenever she had family come visit. The book is very clearly written from an American point of view, and whether that's for the book's benefit or otherwise is more up to the reader, I think, though she does try valiantly to straddle the fence.
My main issue with the book originates from the author's lack of focus, which is quite (in the British sense) spot on. On a chapter that was titled "Proper" and with me expecting an entire chapter on the differences of, say, British/American etiquette or some such, the chapter takes a careering turn to talk about the differences in our breakfasts, which is fine, just not exactly what the chapter starts with or what it was advertising by having a title that said "Proper" and not "Brekkie," for example. It's like if right now I started talking about an entirely different book, like the Language Instinct by Steven Pinker which is distantly related due to discussions of linguistic differences, but ultimately not what anyone was expecting when reading a review for That's Not English by Erin Moore.
Still, it's a thoughtful, if quick, book written by someone who was clearly genuinely intrigued by the differences of her two countries.