Wizards could use the Blessed Book as a PDA in terms of putting in new spells without spending money or XP. When they want to change a spell, they simply erase it and put in the new one.
For Bards, I meant more of the scroll idea where they could take existing arcane and divine scrolls and inscribe them into the Blessed Book as a PDA where it becomes a one-time use thing. Once the inscribed spell is cast from the Blessed Book, it is gone and must be replaced with another inscribed scroll. This gives the Bard a portable, organized and convenient magic arsenal where he could simply open a book and cast a spell of one of its pages the same way he would a scroll. Obviously, used this way, the Blessed Book would become valuable to anybody that can read and cast from magic scolls.