Thanks to Mary Grace Bertulfo, via Perla Daly:
BABAYLANISM IN NEGROS: 1896-1907 by Evelyn Tan Culamar (New Day Publishers, 1986), which is academic in nature, covering the role of babaylanism in Negros history.
And THE SOUL BOOK by Francisco R. Demetrio, Gilda Cordero-Fernando, and Fernando N. Zialcita (GCF Books, 1991.), which is more artistic and anthropological. There are some wonderful drawings by Roberto B. Feleo that show Filipino mythology and spirituality; I think you'd really dig that one.