I’ve recently come across two BBC documentary series, The Genius of Photography and The Genius of Design. Both are absolutely brilliant, serving as visually-rich and engaging encyclopedias of their respective subjects. Best of all, they have a focus on the content, rather than some smarmy host. They are also packed with interviews with key people who actually matter, so will surely serve as excellent historical reference.
Be sure to look for them at your favourite DVD retailer or purveyor of online video