These books may be of assistance:

The Silk Road: A New History, by Valerie Hansen: http://books.google.com.au/books/about/The_Silk_Road.html?id=0LYDf02jgdAC&redir_esc=y

Central Asia in World History (The New Oxford World History), by Peter B. Golden: http://books.google.com.au/books/about/Central_Asia_in_World_History.html?id=PtT5p-6V5FcC&redir_esc=y

Religions of the Silk Road: Overland Trade and Cultural Exchange from Antiquity to the Fifteenth Century, by Richard Foltz: http://books.google.com.au/books?id=czVRPa7GxrUC&dq=Religions+of+the+Silk+Road:+Overland+Trade+and+Cultural+Exchange+from+Antiquity+to+the+Fifteenth+Century,+by+Richard+Foltz&hl=en&sa=X&ei=UYOpU4asIoWGkQXG94GIDw&ved=0CB0Q6AEwAA

Additionally, these online sites may help as well:

http://www.silkroadfoundation.org/newsletter/december/new_discoveries.htm

http://silkroadsogdians.blogspot.com.au/

Hope this is useful to you.

Etienne de la Vaissiere_ James Ward (Translator) (Author)-Sogdian Traders_ A History (Handbook of Oriental Studies) (Handbook of Oriental Studies) (2005)

Haven't read it but I found it in the bibliography of Xinru Liu's history of the Silk Road, which briefly deals with the sogdians as well.