both books are in their 7th edition (what a coincidence....) it's not a case of one book treating each topic more deeply than the oher, it's a case of one book having certain topics and the other book have the exact same text (same words - word for word) plus extra chapters. And no the difference is not 50%, you pay 25% more to get 80% more content, so there is no justification on price, it simply means two identical books are being sold, one happens to have more topics covered, now if each book was book 1 and book 2 with no duplication but a sensible division given that the "big" book does run into over 700 pages versus the over 400 of the "small" book then I would not mind paying £20 per book and buying the two if it meant I got more manageable books at 350-400 pages instead of nearly 800 pages all in one book.