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The 40-year-old filmmaker, who was born in Bangkok but grew up in the country’s northeast Isan region, says Uncle Boonmee was inspired by a book he discovered at a Buddhist temple near his family home in Khon Kaen. “It was this little book that a monk there had self-published and distributed to the villagers for free,” he recalls. “The monk wrote about this guy who came to the temple, who claimed to remember his past lives.”
The good news is that the book seem to have a Buddhist Monk creative common license and might be legal to publish online :). It's in Thai and the English title is only a translation, it was never actually translated. The library that have the best chance to have the book is in Khon Kaen, Thailand. If you can contact Apichatpong Weerasethakul, he might be able to scan it .