Do you have access to Bastion Press' Arms and Armor? If not folks here can help you wing it:
The book provides a level progression for intelligent items. Reason for destroying it: It's your campaigns BBEG!! As it grows in power, it takes over its various 'owners' (increased Ego score). Don't let them in on it until the right moment. Imagine th looks on their faces when they realize that all of those evil wizards they killed were actually controlled by the book! Cliche, but still effective if timed right.
After that, let it slip that when it reaches 21 level, something bad happens (It becomes an avatar of evil, it opens a gate to all of the levels of the Abyss at once, etc). They have to truely destroy it before that happens.
General progression:
Level 1: Two attributes at 2d6+5, one attribute at 3d6, semi empathic, one primary ability
Level 2: +1 to both primary attributes
Level 3: Empathy
Level 4: One additional primary ability
Level 5: +1 to both primary Attributes
Level 6: Speech
Level 7: One additional Primary ability
Level 8: +1 to both primary attributes
Level 9: Ability to Read spoken languanges
Level 10: +1 to primary attributes
Level 11: Telepathy
Level 12: +1 to secondary attribute
Level 13: One Extraordinary power
Level 14: +1 to primary attributes
Level 15: Ability to read magic
Level 16: One additional extraordinary power
Level 17: +1 to primary attributes
Level 18: One additional Primary ability
Level 19: One Special purpose
Level 20: +1 to primary attributes
Also, the items gain the same exp as their owner. They don't 'take a share'. You won't have to explain why they're getting less exp, although that could make or an interesting mechanic by itself.