Tomes, a relatively recent addition to the game, give players the ability to obtain skills they have already unlocked. This allows players to comfortably expand their skill list, and makes tomes a good thing to have.
Tomes drop in hard mode, and are predetermined by the class of the monster that drops them.
However, a problem arises where the distribution of classes across the worlds of Tyria, Cantha and Elona are not even. Warrior enemies are exceedingly common, where monk enemies are rare, and ritualist enemies rarer.
As a result, the distribution of tomes across the classes is not even. A person who plays a warrior can readily obtain the tomes to unlock his entire skill list, where a person who plays a ritualist does not have that opportunity.
A ready solution might be to randomize tome drops in hard mode. Thus, a warrior might drop a tome for any class. This would expand the options for classes already scarce in PvE and facilitate trade between players.