It's not directly music, covers commercial art as a whole: Everything is a Remix.
I found myself watching a 4 hour Tom Petty and the Heartbreakers documentary on netflix last month. I know way more about Tom Petty than I will ever need to, but I think it had some good insight on the production side of things.
Synth Britannia is really excellent. It's better than most documentaries I have seen about scenes and periods of time. Of course New Wave and 80s -90s electronic music are very dear to me. Interesting enough, Kraftwerk was the band on top at the time in club ticket sales. Kinda funny, as we in the US considered them to be very underground.
"The Sound of Progress" is a pretty interesting documentary about the UK alternative scene with great interviews from COIL, CURRENT 93 and others. There's a really bad quality rip of it on youtube. Seem pretty low budget but like I said the interviews are really great and its substance over style in general. Definitely worth a watch!!