What a monumental work! But it is awesome in its scope, depth and breadth. Although the entire concept of historical linguistics has now been rendered obsolete by Chomsky and others, nevertheless, it was a fascinating journey across words & phrases that unveiled the past of us, the Bengalis. Highly recommended, if you can dare to jump into this very-very deep book.
One of the best resources about the linguistic history of Modern Bengali, and also to some extent other New IA languages. It's an old work, so some parts may be understandably obsolete; but still as a whole quite insightful and comprehensive.