I can recommend to you Karma Phuntsho's The History of Bhutan for a very detailed analysis of this. But also check out these threads which have broached the topic:

• What stopped the British Empire from invading Nepal and Bhutan when they had most of South Asia under control?

• Why were Nepal and Bhutan separate from the Indian Empire?

• As I understand, the Princely States in 1947 were given 3 options: join India, join Pakistan, or remain independent. Were Nepal, Bhutan, and Ceylon given those options as well or were they not expected to join either state?