Into the Heart of Borneo by Redmond O'Hanlon — O'Hanlon's account of a journey into the interior of Borneo with the poet James Fenton is one of the funniest travel books ever written. It says almost nothing specific about Kota Kinabalu, but it captures the jungle, the rivers, and the sheer improbability of Borneo travel with a vividness that puts you in the right frame of mind before you arrive.
The Lord of the Flies has nothing to do with Sabah — but The Sea and the Jungle by H.M. Tomlinson, a lesser-known classic, covers the experience of a British traveller in early-twentieth-century equatorial waters and evokes the sense of wonder and disorientation that still applies.
Stranger in the Forest by Eric Hansen — Hansen walked across Borneo on foot in 1982 with no guides, no GPS, and no experience, relying on the hospitality of Penan and Dayak communities. The resulting book is a remarkable account of jungle survival and of the communities that know the forest as a home rather than an obstacle. Essential reading before any trip inland from KK.
Malaysia, Singapore and Brunei (Lonely Planet) — practical rather than literary, but the Sabah chapter is unusually well-researched and contains specific, current information on the climbing permit system for Mount Kinabalu and on wildlife reserves that the older sources miss.