As the title states, the tomes for learning powers can be used inside dungeons. In particular I found it out while going through Kitsune Den and misclicking on a tome I just got while going through my inventory. The process of learning the power from the tome goes as normal, skipping the set amount of time - which can be a nasty surprise, since it leads to losing the timed buffs.
As an aside, I think the fact that learning powers from tomes consumes a whole day should be relayed more clearly to the player, either through their item descriptions or through a separate system message that will require the player's confirmation.
As an aside, I think the fact that learning powers from tomes consumes a whole day should be relayed more clearly to the player, either through their item descriptions or through a separate system message that will require the player's confirmation.