Search Google Books for full books and previews. Also find links to buy it as an e-book or a physical copy.
The amount of text you can see for a book is listed in each result:
Additionally, you may see:
Open Library attempts to have a record of every book ever published. When available, it links to full e-books.
Check Only show e-books to include only results with some type of electronic copy available.
After a search, look at the icon by the title to see what your options are:
Click the book titles to see more information and choose an e-book format, if available.
All e-books from Project Gutenberg can be freely downloaded in formats such as ePub, Kindle, and HTML. The collection is mostly fiction and older books, but you can also find classical texts from theorists. You can browse the book catalog on their site or use the search box to enter search terms.
Find full books at HathiTrust that you can view on screen. Some may also be downloaded as PDFs. You can search for books by title, author, subject, and so forth, or can you find books by searching the full text:
The Online Books Page from UPenn lists over one million free books. You can search by title and author, browse by author, browse by title, and even browse by subject.
Though a small percentage of the Library of Congress collections are in digital form, the number of digitally available items grows every day. Most e-books from the Library of Congress can be found in the in its online catalog, which allows browsing by topic or alphabetically. Library of Congress publications such as annual reports, illustrated collection guides, Federal Research Division country studies, and a history of the Library of Congress are available as e-books.