Ekai Kawaguchi was a japanese zen monk who around 1900 walked to Tibet. Apparently he was interested in old buddhist manuscripts. The fact that more and more western nations were interested in the Big Game between Britain and Russia might have been a factor as well. It is a fascinating book.
"Three years in Tibet"

Francis Younghusband. A British officer, as British as Big Ben, led a military expedition into Tibet, saw the light there and became a near mystic.
"The British Invasion of Tibet."

Read thye other side of the medal at: http://www.freetibet.org/campaigns/younghusband.html

Other leads to find reading:
Anne Louise Strong
Allexandra David Néel

There is a very good english book about the first westerners who tried to reach Lhasa. I do not find the title. One of them was the dutch missionary Rijnhart. There was also a belgian priest named De Deken. A rather sad book about people with a slight obsession.

There are many books about the Chinese presence in Tibet and the cultural oppression of the Tibetans. Read them before you travel.

Have a nice trip.
References :
http://www.tibet.ca/en/wtnarchive/2006/4/19-4_5.html