Hi. Yes this question has cropped up a couple of times. The List of Special Items thread http://www.mongoosepublishing.com/phpBB2/viewtopic.php?t=41637&highlight= has a bit about it.
People seem divided. I like to think that even from book 1 you may leave equipment with the king and others but other members don't agree because the monastery is still a ruin.
Book 6 states for the first time that excess coins may be left at the monastery, as far as I know.
Book 8 states a limit on Special Items as 12 maximum but others may be left in Safe Keeping at the monastery: So in theory there is no limit to the number of Special Items you can carry from books 1-7.
Backpack Item and Weapons are not really mentioned much so it is up to you. I always thought of Safe Keeping as a place of storage but not really retrieval but it is a reaonable point.
So I would suggest there is opportunity to store/retrieve items from Safe Keeping in all the books EXCEPT between Books 10-11, 17-18, 21-23, 27-28.
Other books although instantaneous involve the monastery or individuals equipment MAY be entrusted/retreived from including 1-2, 8-10, 11-12, 19-20, 24-25.