I don't think the syllabus has changed for chemistry over the past year or two. I used the third edition of Conquering Chemistry in 2004 and it covered the syllabus quite well. Although, the definition of eutrophication in the third edition is incorrect, so don't use that definition (you'll encounter this in the third module). I'm not sure whether the author has amended that or not in the fourth edition. I'd get whatever is cheaper, because I doubt there would be any major changes between the two editions.