Its more that Tome stuff is balanced to Core spellcasting, which is broken. Tome stuff isn't as broken as Core spellcasting, but its generally as powerful as non cheesy spellcasting.
If you have a mainstay spellcaster in your party (Druid, Wizard, Cleric, Sorcerer), then its balanced. If somehow your entire group is fighter types, then its overpowered. But that's because fighter types in general are very underpowered (unless they are Warblades, Crusaders, or Swordsages).