I don't why but my Shai character has an option to learn this knowledge summoning Black Spirit.

This knowledge can be acquired again and again by speaking with your black spirit as long as you have deleted it. A good way to get S is to simply delete the knowledge at Annolisa Rosie then speak to your black spirit, rince and repeat.
Alternativly, the NPC Grusha near Velia can grant you the knowledge if you satisfy with the condition (which include story quest); this method of acquisition is way more annoying than the previous one tho.

For me nothing works to get it multiple times. Deleted the knowledge. Created a new character and got it from [Balenos] Sands of Time. Deleted it again and still cannot get it from black spirit or Grusha.
Guess PA broke this with removal of boss scrolls?

Edit: for clarification, I understand how to get it via deleting and doing quest again, but it should be available from NPC, which is broken for now.