Recently, no doubt as a result of teaching economics, I've become interested in medieval economics. Actually, it's always interested me - particularly coinage and bills of sale, because it ties into modern negotiable instruments when I teach it, but I hadn't attempted any systematic study.
Currently I'm reading Power and Profit: The Merchant in Medieval Europe by Peter Spufford. This is a big gorgeous book published in 2003 in the US. Spufford is the author of the seminal work on medieval currency, Money and its Use in Medieval Europe.
Power and Profit is more readable than its predecessor. It is lavishly illustrated and written with a little more attention, perhaps, for the satisfaction of the reader. There are stunning photographs of medieval artifacts I have never seen in other sources.