In both The Lord of the Rings and The Silmarillion, it is suggested that men can learn to use magic. Quotes demonstrating this follow: (from The Lord of the Rings):
The Lieutenant of the Tower of Barad-dûr he was, and his name is remembered in no tale; for he himself had forgotten it, and he said: 'I am the Mouth of Sauron.' But it is told that he was a renegade, who came of the race of those that are named the Black Númenóreans; for they established their dwellings in Middle-earth during the years of Sauron's domination, and they worshipped him, being enamoured of evil knowledge. And he entered the service of the Dark Tower when it first rose again, and because of his cunning he grew ever higher in the Lord's favour; and he learned great sorcery, and knew much of the mind of Sauron; and he was more cruel than any orc.
(from The Silmarillion):
Those who used the Nine Rings became mighty in their day, kings, sorcerers, and warriors of old. They obtained glory and great wealth, yet it turned to their undoing
This makes me wonder... are there any other instances of men who have the ability to learn magic? Also, why don't all men learn magic? It seems to me that Sauron would have had a much harder time dominating men if the men he sought to dominate were sorcerers, themselves. Additionally, if magic is something that can be learned, what, then, separates a maiar or an ainur from a mere man?