The tome of xp 3rd ed is one of the rewards from The Branches of Darkmeyer quest. It starts as Tome of xp 3rd ed (3) whereby each time it is used, for a maximum of 3 times, it gives 50,000 experience to a skill of the player's choice, which must be above level 60. This tome in total grants 150,000 experience when fully used. As it cannot be deposited in a bank, destroy it and retrieve it from Veliaf if you intend to use it later. If retrieved after The Lord of Vampyrium, you can get this tome from Ivan in Paterdomus.
Giving twenty times as much in experience, this tome gives is a huge leap up from its predecessor, Tome of xp 2nd ed.