Researching can be such a struggle. Where to you start? What exactly are you looking for?
The upcoming posts are some of paths and web links I have followed and found interesting.
Posters seem to be rather varied in there design, mainly dependent on their purpose. Music posters are fascinating to look at, as they are frequently focused on the text, as the music itself can not be clearly represented in an image. When images are used they are often tied quite closely to the text.
Because they are not attempting to represent a certain visual style many are very bold and bright.