I read a little, and now there are two versions of Li Fan and Li Wang. I have read a little, and I feel that the difference is not very big, because it is enough to review the whole book as long as all the knowledge points are covered, and both books are ok, but I think quite a few people use Li Fan's, so you can consider this. Li Er's book is really good. It was used in all three exams, but it has not been published yet. You'd better use the latest books. My personal suggestion. As for the problem, 660 is highly recommended. It's a small question, but it can be used as an answer. Good. I suggest doing it more often.