In addition to the packages that Cornell subscribes to there are many sources (like Google Books) that provide access to free ebooks in the public domain or to previews of ebooks still in copyright.
HathiTrust Digital Library is a digital preservation repository that provides long-term preservation and access services for public domain and in copyright content from a variety of sources, including Google, the Internet Archive, Microsoft, and in-house partner institution initiatives.
Due to copyright issues and the preferences of book publishers, access to the content of many digitized books in copyright (published after 1922 and not in the public domain) is limited. In particular, Google Books and Amazon provide a Preview mode that allows only access to selected pages of an ebook. Access to some ebooks is limited to the table of contents or the title page only. For many ebooks for sale, Amazon allows no access even at the Preview level. On the other end of the spectrum, some university presses allow full access to the content of their books online, even if they are for sale in Amazon.
One of the most powerful functions available in electronic books is the ability to searching the entire text of a book or books and locate words or phrases within them. Many printed books, especially scholarly books, have manual indexes at the back to help users find the specific location of the information they need. Digitized books allow readers to search every word in the content of the book.
But, caveat emptor: the accuracy of the search depends on the accuracy of the method used to digitize the content. Digitizing software (OCR, or optical character recognition software) is not 100% accurate at recognizing the words in the text.
And, if only portions of the ebook are offered in full-page view, your word searches may be limited to only those pages, not the entire book. Compare mishka in Penguin's 2001 edition of Anna Karenina in Hathi Trust and Google Books.
Nevertheless, searching ebooks is a very powerful and potentially time-saving capability of ebooks and ebook collections.