I've tried a few Thai ebooks using a Thai font called Angsana. The consonants are great. The higher and lower vowels problems that normally expected are rare. Overall, they looks fabulous but, in fact, unreadable.
At the end of every lines, a few words keep disappearing. I've used ePub and fb2. The results are exactly the same for both formats. Someone recommends about making PDF with big font size and smaller document size but I haven't try this way.