You can start activating after PRing the book the first time. PRing more often gives experience in the process, and, as often said on this DB, PR whatever you can, as often as you can.
Activation is normally not a "one time thing". Each time you pick up the book and do something to it/with it, that means its content
, you are activating. If you read your books four times, that counts as four activations.
For experience, you can for each PRing state a different goal: To get the atmosphere, to get the subtle characteristics of the characters in the book, and so on.
Bye,
Jens