Welcome to the forum rustorod!
I have some RQ, Hawkmoon and elric books. If you are playing just Elric, I would recommend monsters, but these would be generic and gloranthan ones, if you are looking for addtional 'canon' elric monsters, than they will be in upcoming elric suppliments.
Spellbook would probably clutter it up. At least with monsters you can rename or redescribe something, changes its name or say it is from another plane in the multiverse, or something of chaos.
Spellbook would not be great. Granted there are some sorcery spells in it (I think, cannot quite remember!) but a lot of it is legendary magic (might be useful), rune magic (does not exist in elric, but could if you did work, but I think that would take away from the game). There are also some divine spells. They could be used by some cult from elric, but are not really part of it. Could work.
Spellbook could work but it is a tad expensive. Starting out on a budget, no. Check it out, read a bit of it in your lgs if they have one or ask a freind what they think. I would not recommend it unless you feel you need A LOT more magic. Even then though, there will be some work invovled.
Hope that helps!