Encyclopedias, dictionaries and handbooks are important sources of information. Use them for definitions and explanantions of key concepts of a topic. They can provide important background information such as the theoretical foundations and academic terminology for a topic.
They are a very good starting point for reseaching an essay topic.
Print format reference works are held in the Reference Collection of libraries. However some are online eg Oxford art online (see link below)
Be judicious in your use of free reference material on the internet. For example Wikipedia is a collaborative online encyclopedia to which anyone can contribute. Information in Wikipedia is not necessarily accurate nor authoritative. However, Wikipedia can be a starting point to identify keywords which relate to your research topic. Do not cite or reference Wikipedia in an essay.