Good questions. The example of walking between cars is a little confusing because physically we imagine the person in the middle of two cars at any one time. So I would use "between" in both cases. On the other hand, if "he walked among the children", this gives us the idea of a more disorganised scene. You could say this : "Among all the cars I have driven, I liked the Volvo S70 the most." 3. We know precisely which pair or specific group of books you mean. 4. We know the general group of books you mean but not the specific ones within the group.