The Malay Archipelago by Alfred Russel Wallace (1869) — the naturalist's account of eight years travelling through the islands of Southeast Asia, including the Malay Peninsula. Wallace was developing his theory of evolution here simultaneously with Darwin, and his observations of birds, beetles, and orangutans are still vivid. Not specifically about Langkawi, but essential context for understanding the region's extraordinary biodiversity.
Kampung Boy by Lat (1979) — the Malaysian cartoonist Lat's graphic memoir of growing up in a traditional Malay village in the 1950s. Funny, warm, and illuminating about the way of life that shaped the country before mass tourism. Widely available in Malaysia and worth reading before you arrive.
Malaysia and Singapore: The Rough Guide — the best of the practical guidebooks for this region, particularly useful for understanding the ethnic and religious complexity of Malaysian culture that can be invisible to the casual visitor on a beach holiday.