Having looked at the FAQ I hope this isn't considered too vague, but I'm really interested in what have been the largest and most prosperous trade routes in history, land or sea.
I've put before 1800 because I am thinking about potential topics for my dissertation, and the thought of studying trade routes sounds very interesting and something I'd like to more about - and generally my interest in history is before this date.
I know the Venetian Republic had a prosperous trade before its collapse, and Britain of course had a large network, but I'm sure there are more vast trading routes that I am not aware about.
The Trans-Saharan Trade Route is largely attributed for assisting the spread of Islam. The gold for salt trade was one of the largest trading networks during the Islamic Golden Age, ~7-12th century AD. Very interesting to compare trade routes and the spread of religion and culture.
http://www.metmuseum.org/toah/hd/gold/hd_gold.htm
http://www.historyhaven.com/APWH/unit2/africa_and_its_golden_age_of_emp.htm
http://www.oxfordbibliographies.com/view/document/obo-9780199846733/obo-9780199846733-0009.xml
Take a look at the Spice trade route through the Indian Ocean, as far as I know it's one of the oldest trade routes, and probably the most profitable. Keeping in mind you are writing a dissertation, the Spice route would be fantastic to look at. Not only is it interesting economically but it provides a battleground for empires and a way of spreading religion (specifically Islam)
Check out this book: The spice route: a history for an overall look at it.
My personal favorite is The Ottoman Age of Exploration, this book details the Ottoman and Portuguese conflicts in the Indian Ocean over the Spice route.
If I think of any other books I'll edit them in.