Reference sources (encyclopedias, dictionaries, etc.) are useful in three ways:

• They provide an overview of topics, which can help sort out the shape of the forest before you go after individual trees.
• They often identify the core writers in a topic, the voices that are foundational or cutting-edge:  use any bibliographies you find to help develop your own.
• They save time when you're looking for quick information about topics that you don't need to explore in depth.

Consult them at the beginning of your research, and when you run across unfamiliar concepts or topics.