A grammar-book, yes. The subtitle is The Grammarphobe’s guide to better English in plain English. And while I think that my English isn’t half bad, for an non-native speaker, it can always be improved. So far, the book’s not told me much that I didn’t already know, but it did so clearly, and with humour. (Though titling a sidebar “Metaphors be with you” is a bit much.)
I thought it to be a clear and concise book on the intricacies of the English language, with a nice chapter on email (I got the second revised and expanded edition). You do need a high tolerance for puns, since the writer is very fond of them. She includes sections on clichés, dead grammar-rules (“Never split an infinitive”, “Don’t start a sentence with ‘And’ or ‘Then'”, “Don’t end a sentence with a preposition”,etc.), commonly mixed-up words, etc. It’s a nice addition to say, your copy of Elements of Style. The book is worth checking.