English Grammar in Use
Raymond Murphy · Cambridge University Press
The most widely used English grammar book in the world, and the format is why: one grammar point per two-page spread, explanation on the left, exercises on the right, answers in the back. You can use it for fifteen minutes without committing to a course. For Chinese speakers it is especially good on the two areas Chinese gives no help with at all — articles, and the tense and aspect system — which between them account for most marked errors.
What it does not do: It is a reference and a drill book, not a course: no reading, no listening, and nothing that teaches you to produce a paragraph. The exercises are fill-in-the-blank and will not prepare you for a speaking test.
Free here first: Free English grammar quiz