"Duh, you just read them." I certainly suppose so, but how? where? Right clicking on them does nothing. The DoD wiki does have an article on them, but it merely lists the possible tomes, without explaining how to "activate" them.
Tomes are merely off-hand weapons that you can use for additional stats. So to 'activate' them, you simply equip them in your off-hand slot and you will gain whatever additional stats they have.
Eg: The "Little Black Book" will give you 2 additional Necromantic damage and 1 Necromantic damage resistance. Plus any additional stats should it have more enchantments.