To start at the very beginning.
One website that has save my butt many time when writing papers and researching for shows. Has been The Costume Gallery’s Research Library.
With over 5000 web page and images this resource keeps on giving. You can browse the titles of the books and articles but to gain full access, a subscription fee for a whole month is needed. Well worth paying to enter the Aladdin’s Cave of rare and vintage costume books online.