The Tesseract by Alex Garland — Garland's follow-up to The Beach, set largely in Manila and the Philippines; dark and atmospheric, it captures the underside of Southeast Asian tourism culture that Boracay's White Beach deliberately stands apart from.
On the Road in the Philippines by Monique Wilson — travel writing tracing different corners of the archipelago; puts Boracay in the context of the broader Philippine experience rather than treating it as the only destination in the country.
Noli Me Tangere by José Rizal — the great 19th-century Philippine novel that galvanised the independence movement; essential reading for understanding the country's history and why the Philippines is the place it is today.
Reef by Gyan Prakash, or alternatively a good coral reef marine life field guide — the diving and snorkelling around Boracay and the Visayas rewards identification of what you're seeing; a regional reef fish guide pays dividends in the water.