Tomes go in weapon slots and do no damage, instead giving large stat boosts and powerful bonuses including buffing skills/talents or even adding ranks to talents. Tomes can be fitted in offensive (actively wielded) or support (bow/arrow) slots.

Reasons why you might give up a regular equipment slot for a tome:
--Stat bonuses
--Unique bonuses on specific skills and talents, randomly generated for each tome. Examples include: "Frost bolt travels 100% faster and does 20% more damage", "Healing bolt chains to an additional ally", or "Flame sentinel has its cast time removed".
--If you give up an active weapon wielding slot, the stat bonuses increase and extra talent affixes on the tome become active. This give additional ranks to various talents, including raising them over their normal maximum rank. Raising a talent with only 1 rank over its limit generally adds +40% power (and +40% health for summons) per extra rank, which means that you could dual-wield tomes and theoretically have a fireball that does 80% more damage than normal at the cost of giving up melee and wands. Alternately you could use tomes to give you top-tier talents deep in class trees to avoid having to spend many talent points on the way to unlock them.

Tomes have a short warm-up period (a few seconds) before active talent bonuses (for holding tomes in actively wielded equipment slots) become active. This prevents degenerate usage cases where someone will have a dual wand set and a dual tome set, then use a macro to switch to the tome for the bonuses, throw a fireball or something nasty, and immediately switch back to the dual wand set with no effective penalty for using the tome..