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oints: (Simple or Detailed)
RULEZ 
ON THE INTERNET!
Demode Couture (There are so many links I could post that are already compiled and organized beautifully on Kendra VanCleave's site. Absolutely worth your time.) [link]
The Costumer's Manifesto (cluttered but deep, lots of good links hidden away in the fray) [link]
Wikipedia (surprisingly useful! Great for when you're trying to initially pin down a period.) [link]
For some unadulterated extant dress porn, Tumblr is a fantastic point of reference:
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BOOKS!
Some great overview/survey books:
A History of Costume [link]
What People Wore: 1,800 Illustrations from Ancient Times to the Early Twentieth Century [link]
What People Wore When: A Complete Illustrated History of Costume from Ancient Times to the Nineteenth Century for Every Level of Society [link]
Fashion (Taschen 25th Anniversary) (amazing extant gowns) [link]
Any and all of the "fashion in detail" books:
Nineteenth Century Fashion in Detail [link]
Seventeenth and Eighteenth-Century Fashion in Detail [link]
Fashioning Fashion: European Dress in Detail, 1700 - 1915 [link]
Underwear: Fashion in Detail [link]
Dress in Detail From Around the World [link]
For sheer variety during the Victorian/Edwardian eras, any photographic books or collections of fashion plates are great:
Victorian and Edwardian Fashion: A Photographic Survey [link]
Victorian and Edwardian Fashions from "La Mode Illustree" [link]
Victorian Fashions and Costumes from Harper's Bazar, 1867-1898 [link]
I'm sure I have more (I know I have more, in fact), so I'll try to keep updating this on a regular basis.
-C
ahem, apologies
vintagethimble on tumblr also has some good refs and even a decent collection of period accessories. Most all the photos are of surviving, museum-quality garments on manikins or historical recreations on models posing for the camera (front and back in many cases).
[link] is another one that's got a MASSIVE collection of images organized by a variety of criteria.
If ever you come across a good pattern catalog (fabric prints and designs, not garment construction) that's another good ref to have because I've seen people design "historical" Cambodian-style costumes....but the patterning on appropriate parts is a fleur de lis pattern.
Maybe I missed it in your list, but there is also the Metropolitan museum website, often with multiple views, and with zoomable, high-def photos. Alas, the site used to have much better search tools (unless I just don't know how to use it now, less user friendly if nothing else).
[link]
Hi, Just wanted to let you know about my blog. I'm collecting images of extant garments that aren't in museums to keep a record of them for research purposes.