Arcane Tomes can be obtained by farming them in Burning Crusade Classic Gold Notes: These items are dropped by the same mobs that you farm for Sunfury Signets, so you will be co-farming these items with them.
Following on from the section on farming Sunfury Signets, the list above will appear to be somewhat recognizable. The mobs you farm for Arcane Tomes are exactly the same as the mobs you farm for gold.The best course of action is to follow the above instructions for...