Well... depends which epoch, era, and branch? Please, elaborate.
Books that still apply about tactics, strategy, etc:
All excellent books!
One important thing to keep in mind is the generally static nature of strategy. Most all of the strategic concepts that were true in the era before firearms still apply today.
In addition, many tactics have only changed in the details. As an example, slipping a small, mobile force (horses) through the lines to go ape in the enemy's rear is just as important today as it was centuries ago.
Just the details have changed.
And using terrain to shut down the enemy's strong points? Or using a valley to constrict infantry for an ambush? Examples of these exist from 2500 years ago. Yep, Two thousand, five hundred. And flanking? Prehistoric, no doubt. Organized pack animals such as wolves do this fer cryin' out loud!
From what I understand (and I'm a rank amateur) many fundamental concepts of strategy have changed little compared to the changes in tactics. But if you are going to play a Civil War game, you need to understand how tactics applied in the day, or else...