If you want to understand deer better, the best I've found in almost 60 yrs. of looking is The Deer of North America by Leonard Lee Rue III.
If you need to understand throphy bucks, those with over four years of age, I also vote for Hunting Throphy Deer by Wooters.
Observing deer in the wild, something I can do in my own yard is fine for general deer knowledge. But, if you want to hunt throphy bucks, the only way to learn them and to do that is to do it. Wooters' book tells you how to get started. E