Using an encyclopedia is an excellent way to come up with ideas for topics. There are lots of subject specific encyclopedias that you can just browse to get ideas. It's kind of like going to the grocery store when you're hungry.
Encyclopedia articles tend to be short and easy to read but they give you a lot of information.
Encyclopedia articles will give you ideas for search terms -- key people, places, or dates.
Just remember that you won't be citing the encyclopedia in your paper. It's for you to find a topic or get background information.