Go to the lady that resets knowledge. Look to HER right and see the "altar" between the bookshelves. There should be two books there; one that contains this knowledge as well as "Impregnable Tactics and Strategies".

In the library face Annolisa Rosie. Directly left of her are three pedestals. Two of them have books and one has a candle. One of the books has this knowledge.

There are two books on this podium "Impregnable Tactics and Strategies" and "How to Pep Up Your Soldiers". It took forever for me to realize there where two on this podium as I only saw the first one. Anyway, I hope this helps someone as well.

Black Desert, Кальфеон и валькирии (Как вдохновить ваших солдат)
https://www.youtube.com/watch?v=M6gq6RY1h-0