There are different ways to make a region a geographical unit. It can be seen geological, morphological, climatological, cultural, ethical or political for example. In the case of South-East Asia the routes and description of travelers that passed though the area between China and India seem to have a large role in defining it as a region.
Thomas Suarez book on the “Early Mapping of South East Asia” describes South East Asia through the eyes of old maps and sailing instructions. It is a fascinating and deep introduction into routes, regions and views put together from partly conflicting sources. This book is on history, geography and an adventure to read at the same time. It is obvious that Suarez is a real conaisseur and authority on early maps. His interest in South East Asia was awakened when he travelled here on concert tours as a classical violinist.
“Early Mapping of South East Asia” you find in Singapore book shops for 103 S$ or at http://www.amazon.com/Early-Mapping-Southeast-Thomas-Suarez/dp/9625934707/ref=sr_1_1?ie=UTF8&s=books&qid=1218857154&sr=8-1