Technically yellow tagging your book would be a terrible, terrible idea.
Items with green tags can be taken freely. Items with yellow tags need to be turned in at the end of an event--but will be returned at the event following, most likely. Red tagged items cannot be touched. Items without tags are generally worthless or parts of costuming and should be left alone.
Yellow Tag = Stealable.
No Tag = Worthless/not stealable.