"Death is a friend of ours; and he that is not ready
to entertain him is not at home."

   –  Sir Francis Bacon

The necromancer is a a powerful mage (though with less spells than the traditional mages) who can, among other things, raise the dead to aid him in his battle, even raise dead mounts for him to ride. Spirit Speak is the complementary skill to Necromancy. Just like Evaluate Intelligence for traditional mages, Spirit Speak will increase the potency, duration and effectiveness of your spells as well as restore your HP (just like the heal spell. You should also have a mana regeneration skill as you will use up mana to cast your spells. You should therefore have either Meditation or Battle Focus (depending if you're mainly mage or warrior). Since many of the necromancy spells are poison based, Poisoning is also recommended (but not compulsory) as it will make your spells even more lethal.

The necromancy spellbook contains up to 16 spells but you will need to find them first. The empty spellbooks can be bought from any NPC mage or necromancer that normally sells scrolls, spellbooks, runes, etc. You will then need to kill various monsters to collect all 16 spells. The most common creatures to carry them are: liches, lich lords, balrons, white wyrms, ancient wyrms, wyverns, drakes, dragons, ancient liches, zealots of Khaldun, meer mages. You can also find them in MIBs and treasure chests.

You can use the 8x8 method to train your character. However, since you cannot cast necro spells on the tillerman, and casting on yourself would require a lot of healing, the table below based the training on transformation spells.  

Skill	Spells
0	Buy skills from NPC Necromancer
30	Pain Strike
54	Horrific Beast
71	Wither
81	Lich Form
110	Vampiric Embrace

Note that in wraith form, you will be able to recall through a runebook just like a mage without the need of any reagents.