We have seen all kinds of English study kits and books in the market. Michael Swan's "Practical English Usage" is one of the most practical handbook for everyone who uses or wants to learn English for daily usage.
The term "usage" here probably needs a clarification, as people sometimes use "grammar", "mechanic" and "usage" interchangably. Similar as they are, they actually have different meanings (at least if you consult the American Heritage Dictionary). "Usage" here refers to the way a particular word in a language, or a language in general, is used. So to clarify: although grammar is part of the content, this book is not a grammar book in the sense of typical grammar book classified by grammatical points, but rather a usage book classified by alphabetical topics.
What are the differences between at, on and in with expressions of place? Is "unless" the same as "if not"? What are the differences between "each" and "every", between "fairly", "quite", "rather" and "pretty"?
This book is a guide to problems of this kind. It deals with over 600 points which regularly cause difficulty to foreign students (or teachers!) of English. The explanation deal mainly with standard modern everyday British English (the few grammatical differences between British and American English are also described).
Now how important is correctness (for you to buy this book?) Well, I will quote Michael Swan's own comment: "Learners should aim to avoid serious mistakes, and a book like Practical English Usage will help considerably with this, but they should not become obsessed with correctness, or worry every time they make a mistake. Grammar is not the most important thing in the world!"
And I will let him have the last word on the matter.