I have read both the books, Exploring will suck your blood, will make you too much boring and is also outdated as per current trend of CSS exam. I believe discovering is the best book. It contains every thing what is needed to pass composition paper even in accordance with the current syllabus to come up with a good result. I will recommend you Discovering, because it contains a variety of chapters from all the best series in its own form. It is the best book not only for CSS but also for the rest of the general tests.