Reference eBooks, like reference books, include dictionaries, encyclopedias, handbooks, guidebooks, glossaries, atlases, etc.
They are a great place to find a quick answer or get a introduction to your topic.
Instead of being taken to the book, you will be taken to an arricle or entry withing the reference book.
Some contain useful bibliographies at the end of the article to help you expand your research.
The Thrift Library's Reference Ebook Databases - General Topics
All of these databases may be searched within OneSearch by limiting your search to Reference Entries under Resource Type.