Arcane Tomes, along with Firewing Signets and Sunfury Signets, are items used to get reputation with the Scryers in Shattrath City.
This item is dropped by higher level Blood Elves in Shadowmoon Valley, Netherstorm and The Isle of Quel'Danas. Contrary to popular belief, Blood Elves in Firewing Point also drop Arcane Tomes, despite the description of  Synthesis of Power, which clearly states that they drop from higher level mobs.
As a quest objective
Arcane Tomes are given to Voren'thal the Seer in the Seer's Library of the Scryer's Tier in Shattrath City. For each tome turned in, players will receive: