Reformed literature in Thai is very hard to find. If we define the term "Reformed" somewhat broadly, then some Reformed titles may be found at various Thai publishers. A few of those titles don't seem to be translated well, so I can not recommend them with confidence. However, I can recommend the following titles
Published by Kanok Bannasan (OMF Publishers Thailand):
The Holiness of God by R.C. Sproul (currently out-of-print)
Concise Theology by J.I. Packer
Pilgrim's Progress by John Bunyan
Today's Gospel by Walter Chantry
Self-Image: How to Overcome Inferiority Judgments (Resources for Biblical Living) by Lou Priolo
Published by Biblica:
Jesus Storybook Bible by Sally Lloyd-Jones
Available for free online:
The Westminster Shorter Catechism in Thai can be downloaded here.
The Heidelberg Catechism in Thai can be downloaded here.