The CGP range of books are used by my son in a variety of subjects. He enjoys using them as they are clear, easy to follow and have a mild (although sometimes poor!) humour throughout them. I find the Study Books useful to have as it gives an idea around the syllabus they are studying (it is a few years since my O levels) and lets me ask questions once I know what is being studied. The illustrations are good and there are some excellent examples. This is definitely a series I find myself reading for half an hour to "remind" myself of a subject.
My minor gripe is that I would not class the CGP books as 100% useful for study. They lack an amount of detail so you would need a reference book to give more meat to a subject.
Overall, a well presented set of books which are more useful to parents than to students, in my opinion.