The Internet Archive is an amazing resource. I especially recommend its digitized books. When compared with projects like Google Books, the Internet Archive makes up in quality what it lacks in quantity. In particular, their "Flip Book" feature (developed as part of their daughter site openlibrary.org) makes reading books online quite a joy. It recreates the book experience, complete with animated page turning (not as hokey as it sounds--I quite like it), and the ability to flip to a given page based on its position within the book. Add searching and the ability to download books in a surprising number of formats (text, HTML, DjVu, PDF, TIF, JPG), and you've got an impressive total package. Check it out to see just what I mean.
There are a number of interesting books in their catalog, but I'm posting about this one because of its possible historical significance: Eric Reid's Spears of deliverance : a tale of white men and brown women in Siam, published in 1920. It's a novel, and from flipping through it, it looks like typical expat fiction (although the subtitle would tend to give this away already). I'm not a fan of the genre, but this is the earliest example I've ever seen of expat fiction about Thailand. Can anyone provide earlier examples, or leads?
Both Google Books and the Internet Archive have a number of other accounts of historical Siam, if you're looking for non-fiction fare. Search "Siam" in either place for a start. Lately I've taken to downloading them offline as a personal archive, with the idea of creating a website to be able to read and search historical accounts of the country in a unified place. It remains to be seen whether Spears of deliverance and its ilk have actual historical value in this regard, though I'm inclined to cast the net more wide than narrow.