Looking for info on the Medieval Free Cities.

Which ones? The Italian ones? The Hansa? The Low Countries?

If you just want a background, I'd check out one of Cambridge's histories of medieval <blank>.
 
Any and all of them.

I really don't think you have a chance of finding that in any handy package. For the core areas, I would recommend E. Pitz: Europäisches Städtewesen und Bürgertum. He doesn't look too much at the peripheral areas and his study stops with the high middle ages, but it's nonetheless pretty good for so comprehensive a book. But generally, you'll need to look at geographically more limited histories.
 
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