I think an important series of questions to answer would have to do with how you want to think about coffee, and what your questions are. Are you more interested in history of capitalism/commodity flows questions? Globalization and socio-economic injustice? Environmental issues? Food studies? Business history? Cultural history? History of craft and labor? All these would prompt you to look to study under different people or programs. If you are more interested in the topic than the questions, then maybe a masters program in food studies would be right for you. NYU and GMU both have food studies programs with historians on faculty. There are lots of other programs too, but others like at Berkeley and UVM tend to be more sustainability/activist oriented than you seem to be thinking . . .

You might try examining the bibliographies of coffee books/authors you particularly enjoy. Try looking at Pendergrast's Uncommon Grounds to see if there's a particular university/school/professor who continues to pop up.

All good suggestions and exactly what I came here for. I can not say thank you enough.

A cool book you might want to check out is Manuel Moreno Fraginal's classic El Ingenio, translated into English as 'The Sugarmill'. It is a study of how the society, culture, economy, even language, of Cubans was shaped by the production of sugar. Since sugar, coffee, cotton, etc., tended to be monocrops that were cultivated extensively, each product tended to shape the society which rose around it. Its methodology can be used as a model for just about any study and his beautiful writing shows that anything can be interesting if you know how to write about it. It became really influential in Latin America and definitely worth the read given your niche interest in coffee. To my knowledge no one has taken his model and applied it to other basic products. It would be cool to see something similar written about, say, Brazil, which was a huge coffee producer, to the point of having an oligarchical government popularly known as the Coffee and Milk government, due to it being essentially an alliance between cattle ranchers and coffee plantation owners.

What about the degree would allow you to further your career from its current place?

Can you pay for the schooling or will you need to take on loans?

Will your increased salary (if there is one) be able to pay down any student loans acquired in obtaining this degree?

In terms of sources, NHANES has distributed dietary questionnaires to consumers since the early '60s, and questions about coffee consumption have been part since at least 2000.