Sometimes, a useful starting point can be to start thinking about what what questions you are actually grappling with in your dissertations, in other words what are you actually trying to answer or to prove? In order to do this it's worth drilling down and really trying to define your research question. Our Study Advice Team have produced a video on how you might go about doing this. You can watch it here
You may start out with a few books or articles that you have already found, or that were recommended to you by your supervisor. To find additional material you need to search;
When searching for books or articles think about the keywords you are entering into the search box that define your topic.
Literature searching isn't an exact science. There's lots of trial and error involved. If one thing doesn't work - try another, and remember that a search that doesn't yield many results doesn't necessarily mean you've done something wrong. You might be looking into a question that not much research has been done on!
See what results you get and refine your strategy.
If you do find one or two key articles, or book chapters, don't forget to look at the list of references at the end of the article - these can often lead you on to further useful material.