Two books folk might find interesting - both by Peter Hopkirk.

The Great Game (my copy is the 1991 edition)

Quest for Kim (my copy is the 1997 edition)

Hopkirk has written a number of other books on similar topics (such as "On Secret Service East of Constantinople: The Plot to Bring Down the British Empire", and "Trespassers on the Roof of the World: The Race for Lhasa")