I wish I knew! The reference to this incidence in the bio of Campbell by Stephen and Robin Larsen (A Fire in the Mind) just says the following (on p. 84):
Beach, out of her own considerable understanding of Joyce, took the time to initiate Campbell; and heaped him with books that would help him understand the many-layered historical and mythological context of what he was reading. ‘That changed my career,’ he said.”
Not a lot to go on there. Best we can do is speculate, which doesn’t get me very far.
I would like to think Sylvia Beach pointed him toward Freud, Jung, Goethe, and such – but he first read Freud a few years before, in 1925, and his introduction to Carl Jung came the year after encountering Joyce, reading Jung’s seminal work, Wandlungen und Symbole der Libido (later translated into English originally under the title of Psychology of the Unconscious, and today is known as Symbols of Transformation), while learning German and Sanskrit at the University of Heidelberg 1928-1929 (damn impressive – Jung is an ambitious read in any tongue; boggles my mind that Joe was able to read and absorb it in the original as he was just learning the language!). The same holds for Goethe’s work, some of which he also read that year in German.
I thought I had stumbled across a clue on learning young Joe had given a copy of Sir James Frazer’s The Golden Bough to Angela Gregory, his closest friend in Paris, when she left for the United States – but turns out Campbell had cited Frazer in his Master’s thesis, on the basis of which he earned that Proudfit scholarship that paid for his year at the Sorbonne in Paris, and then Heidelberg.
At this point, it remains a mystery, at least to me . . .