I teach and tutor maths, and found this book very helpful and clear. It splits topics into sections and marks the differences between the syllabuses clearly so the student knows which topics they are expected to know.
It explains key facts well, and has lots of examples and questions to try. It also has sample papers for the student to try, and of course the answers are included.
Most importantly, it's correct - I've been quite scared by how many revision guides contain incorrect information!
It has a section on revision tips, which is also helpful and useful.
My students have always found it helpful, and I recommend it for anyone who's looking for a revision guide for AS level maths.