Reading Indonesia: Peoples and Histories. I selected it because unlike many books it wasn’t incredibly skewed to the early modern and postcolonial period. The author makes the interesting point that the Islamicization of western Indonesia and the rise of the great Javanese Hindu kingdom of Majapahit occurred around the same time. This, in contrast to the skein of Indic civilization which had been layered over maritime Southeast Asia for hundreds of years before the medieval period, starting around 500 AD with polities such as that of Kalingga.
As is usual in these sorts of books, it is emphasized that Indian civilization spread through cultural diffusion (in contrast to the fact that though Chinese trade was evident and present early on, the cultural impact was minimal). Any migrations are dismissed as legends, with the possible exception of a few elite religious functionaries.
I now believe this is wrong. I’ve discussed this extensively in the past, but the Singapore Genome Variation Project (SGVP) data set along with more Southeast Asians allows me to illustrate rather clearly the issues. The short of it is that it is highly likely that substantial South Asian ancestry exists within Southeast Asia, and that that ancestry is not just a function of colonial contact (e.g., as certainly occurred in Malaysia).