I am coming back to the game after a long time away
I don't remeber who I got to about the Giants books
I have the following
giants medical science chap 3, chap 2 and chap 1
blueprint giants golem
giants love letter
Blueprint giants golem 3
Handbook Giants Architecture chap 3
These are part of the Exploration of the Giant's Cave. Details are here. (http://lineage2.stratics.com/content/library/quests/quest.php?id=15008) The Love Letters and Doodles are not valuable, just sell them to any NPC. The other books can be turned in when you have a complete set, to Researcher Sobling, near the entrance to the right-most Cave.