eBooks are like, well, books - only electronic. In many cases these digital copies are exactly like the original book.
eBooks tend to fall into two categories - those you read cover to cover and those you access to find an answer.
In the second category, you get in, you find your answer, and you get out. This type of eBook includes dictionaries, encyclopedias, handbook, guides, etc. When using them you may never see the entire book. Instead, the eBook's search engine will only retrieve the relevant articles or passages identified in your search. This kind of eBook acts more like a database than a book.
Compare the different reading / searching-within-text experience of these two eBooks.
An eBook to Read - Uncle Tom's Cabin, by Harriet Becher Stowe.
An eBook to Access - Britannica Online Encyclopedia
NOTE: There are other eBook Reference Databases on specific topics such as literature.
Containing encyclopedias, dictionaries, handbooks, guidebooks, etc. Get in, get your answer, get out.