THE POSTAL HISTORY OF TIBET, by A.C. Waterfall is the most famous book on the stamps and postal History of Tibet.
2 Editions were made:
1965, 1st edition, 174 pages, Hard-cover, dust jacked.
1981, 2nd Edition, 188 pages, Hard-Cover, dust jacked.
The books usually can be found at US-$ 20 for the 1985 edition and slightly higher for the 1st. edition.

The books are called The Philatelic Bible when it comes to Tibet - Absolutely Essential! A must if you are seriously collecting Tibet.

The book covers basically all parts of the Tibetan Postal history as follows:

Prephilately with Dalai Lama and Monastery-Covers
British Indian Field Post Offices in Tibet, from the Border commission in 1903 to the Military Mission / Expedition in 1903 - 1904 up to the British Indian Post in Tibet
Imperial Chinese Post Offices in Tibet from 1904 - 1955
Tibetan Local Post from 1912 - 1960
The Post offices of the Peoples Republic of China in Tibet since 1950
Tibet Mountaineering Expeditions
And for a good measure even the postal History of Bhutan is touched.