Hugh Scott 
In The High Yemen [PDF ebook] 

First Published in 2002. Scott gives a fascinating account of an expedition that took place in 1937 to the Yemen when the country was closed to Europeans by order of the Imam. Ostensibly a scientific expedition, it possesses great political, cultural, and anthropological interest. The tense negotiations which preceded the expedition and the ultimate success assured that this work remains perhaps the most important account ever written of that forbidding land that occupies the southern half of the Arabian shore.