Q:

How does extra damage or extra intelligence come into play?

I understand how the spell damage mechanics work well.
But it seems that even though there are many advantages to have a slow weapon as a wizard (since you spend less arcane power per hit and can benefit from 1 time hits), I would like to know if the following is an advantage towards the high attack speed side:
Having a spellbook gives a flat increase of (i.e.) +3-5 damage.
Does this increase the range of your primary damage to +3-5?
If so, then since this isn't a percent increase, have a high attack speed benefits from such items, isn't it so?
Considering the alternative, I have a shield that could provide 20 intelligence, which translates to 20% more damage by spells.
If my current damage totals to 20 DPS, with 10-30 damage at attack speed 1 per second, then the shield yields 12-36 damage, hence 24 DPS.
Using the spellbook instead of the shield would yield 13-35 damage, hence again 24 DPS.
But if I had attack speed 2 per second, and damage to be 5 - 15 (giving again 20 DPS), my shield would give 6 - 18 damage, thus 12 DPS, but the spellbook would give 8 - 20, hence 14 DPS.
Is this correct or am I missing anything?
Thanks

(edit)
Answer:
As per the video below, it's clear that the flat damage increase can benefit in larger DPS when applied to a high attack speed, making it a valuable option instead of a 2H weapon.

A:

The +damage from offhands, rings, and amulets is added prior the to intelligence bonus being multiplied in.
Many wizards prefer higher attack speed because they cause more hits, and more hits are more opportunities for things that can happen on hit to happen.  (Crits, chance to stun on hit).
eg. this build on reddit uses this trick.

