A small sample of reference works available online as ebooks.
"Reference" is a loose term we use in libraryland to describe those works that have a certain general usefulness to them. These sources include encyclopedias, dictionaries, handbooks, etc. They typically provide broad summary and overview rather than in-depth analysis, so they make great places to start any research project.
Reference works can be found a few different ways. There are standalone reference books (and many reference ebooks) that you can access through the catalog (examples on the left). There are full-text online databases that can search within numerous reference works at the same time (see below).
In all of these, performing broad searches is best: just one or two words, since their contents are typically short overview pieces.