Instantly rewards 10,000 combat fame. Can be used by clicking on the Tome and selecting 'Use' from the corresponding item popup window.
Importantly, the fame applies to whatever combat equipment you are currently wearing, just as if you killed a mob worth 10,000 fame. This item cannot be used to boost gathering skills.
The drop rate of this item is associated with the fame of the mob.
During events that give additional fame (for example +25% of fame for everyone), Tomes of Insight also give more fame ( for example 12,500 combat fame).