This isn't a book but it should give you some key points in the last hundred years so you can look for books about certain events:

http://www.bbc.com/news/world-south-asia-12641778

Here's the "Country Study" for Bhutan: http://lcweb2.loc.gov/frd/cs/bttoc.html

What if there's a book about the history of Bhutan, but it's just good, not actually great? Do you truly demand greatness?