Open Library provides access to many works from other free and open access projects, such as Project Gutenburg, Internet Archive, as well as hundreds of contributing libraries.
There are many other volumes available for borrowing through the Lending Library collection.
Google Books is a wonderful resource for previewing books. The majority of books that Google has digitized are not fully accessible to the public.
Some works are available as "full view," meaning that these works can be read in their entirety online.
Google Books Advanced Search allows users to retrieve "Full view only" books by selecting the appropriate search setting.
HathiTrust is a collaboration between academic libraries and other parties to provide storage and access for their digital collections.
In order to locate the books that are available in their entirety you may use the advanced catalog search.
UC Press E-books provides access to almost 2000 academic titles on a variety of topics. E-books can be viewed online, but there is no download option for offline or device use.
The Internet Archive and Open Library offers over 6,000,000 fully accessible public domain eBooks. This includes a special modern collection of over 500,000 eBooks for users with print disabilities, and a very interesting curated, modern collection for the world at large.
