If fighting and attacking are the main tasks, then the high-level stage (such as 7 books and 8 books) is usually more challenging and fun, because at this stage, the cost of defense upgrade is not very large, it is relatively easy to grab money, and it is basically easier to draw opponents at the same level. If you want to show your strength in the Legend Cup League, then 14 is the real peak stage, because all arms schools will be open to players, so you need to concentrate on the Legend Cup stage.
Generally speaking, a good book on tribal conflict mainly depends on the individual's game goals and interests.