Search the library catalogue and databases
Use keywords: Fashion AND history,
Add keywords such as 70s OR 1970s, 18th, 19th, .or country or region such as Europe, Australia
Fashion AND Australia fashion AND (1970s OR 70s OR seventies)
Search by fashion styles: Fashion AND flares
Don't just look for books about 20th century fashion history. You'll find information about each decade in all sorts of books.
When you are looking for influences on fashion in a particular decade it can be helpful to use the following headings to group your findings:
These headings are convenient but they are not the end of the story. The boundaries between these areas are fluid and changes in one area are often tied to changes in another. For example a political event such as World War I had repercussions in almost every part of life: fewer resources (economic); women working in factories (social and economic); new weapons etc developed (technological changes - which sometimes had an impact on fashion production); new attitudes (social) etc.
The Library has a good range of books that cover musical and social history, the story of bands, and biographies of singers and musicians. These provide background material that might not be found in books on fashion history. Listening to the music from the decade you have chose will also help build up a picture of its culture.