Arms and Eqipment is probably the best one.
There are magic items for various Gloranthan beasties in Players Guide to Glorantha. Legendary Heroes has some powerful items, but I don't rate those particularly highly.
But, if you want one book for magic items then Arms and Equipment is probably your best bet.
Personally, I'm not that keen on a book of spells, a book of magic items and so on as they take the context away from things. Probably suits a generic set of rules, but not a particular setting.
