IMo from personal experience don't bother with revision guides if you want a top grade-they don't give enough detail, however if either you want and average grade or you want a summary to look over, I'd advise against letts. revision express are alright too (not 100% sure whether they do gcse though actually!)If you've got good notes already just use them or use a proper textbook and use your syllabus to check what you need/don't need! hope this helps