Your title talks about "South-East Asia" while your comment talks about India and the subcontinent (which are not part of Southeast Asia) so it is confusing which region you are interested in. I'm guessing you want to hear about books in South Asia rather than Southeast Asia.

Are you interested in the first printed books in South Asia? These probably go back to the 16th century after contact with the Portuguese. Of course, there were countless manuscript books in India going back several centuries before this. By 'first bestseller', which market are you interested in and are you thinking about the first book to become a bestseller, or the bestseller which was written first? These are not the same thing.