For an excellent introduction to Theravāda Buddhism see:

Richard Gombrich,

Theravada Buddhism: A Social History from Ancient Benares to Modern Colombo (London: Routledge, 1988).

For a more detailed account of the formative phase of Buddhism in India see:

Rupert Gethin,

The Foundations of Buddhism (Oxford: Oxford Paperbacks, 1998).

Buddhist Thought: A Complete Introduction to the Indian Tradition (London: Routledge, 2011).

For a more advanced account of Mahāyāna Buddhist thought see:

Mahayana Buddhism: The Doctrinal Foundations (London: Routledge, 1989).

For an illuminating guide to the spread of Buddhism:

अज्ञात पृष्ठ