Hey there. CGP is really useful and I love how the summarize everything so neatly (the textbook waffles a lot and also sometimes includes information not on the spec which can be very misleading) I'd recommend getting it but don't revise completely from CGP notes as they exclude important information. What I suggest is, when revising, use both books and always keep the spec close by if you're unsure about something and want to see whether you need to know it or not