The Spell of the Sensuous by David Abram (1996) — Not about Sumba specifically, but the book's central argument — that indigenous peoples perceive the world through a radically different sensory and spiritual relationship with place — illuminates what you see in Marapu villages better than any ethnography. Read it before you go.
Sumba: Of Horses and Spirits by Jill Gocher and Rio Helmi — A photographic study of Sumba's culture and landscape, with essays on Marapu beliefs, ikat, and the Pasola. Hard to find but worth the effort; a number of secondhand copies circulate among travellers. The photographs communicate what words cannot about the island's visual character.
Indonesian Textiles by Urs Ramseyer — The most thorough treatment of Indonesian weaving traditions, with extensive coverage of Sumba ikat. Essential reading for anyone seriously interested in the cloth. Ramseyer worked with Sumbanese weavers over decades and his descriptions of the symbolic vocabulary of the patterns remain the standard reference.
The Religion of Java by Clifford Geertz (1960) — Geertz worked in Java, not Sumba, but his framework for understanding Javanese animism and the layers of belief that underlie Indonesian religious life helps visitors make sense of what they encounter in the Marapu villages of the west.