[wolverine876]

Tolkien, in addition to being the great fantasy writer of the 20th century, he was also an Oxford professor and the century's great scholar of Old English. And in his domain were the myths of northern and central Europe.

He needed no popular book of the occult; he had read all the original myths in their original ancient languages. There is plenty of scholarly work describing his sources, and he wrote his own essay on it, though it's more conceptual than about primary sources: On Fairy-stories.