I like this book because there are interactive exercises that make learning really easy and effective. I've looked at several grammar ebooks, including those priced around $20. Their exercuses are simply printed out in exactly the same way as the paper version. Dr. Yam's exercises, however, provide responses to correct and incorrect answers, with plenty of relevant information. Even if you are on the move, you can learn something new every few seconds. Does anyone know why there're no interactive exercises on ebooks till now, or am I not looking hard enough?