The Internet Archive offers over 20,000,000 freely downloadable ebooks and texts. These books are in the public domain - no longer (or never) in copyright in the United States.
There is also a collection of 1.3 million modern ebooks that may be borrowed by anyone with a free archive.org account. Books in this collection may be borrowed by logged in patrons for a period of two weeks. You may read the books online in your browser, or download them into Adobe Digital Editions, This Open Libraries project uses “controlled digital lending” or CDL, which allows libraries to loan digitized books to digital patrons in a “lend like print” model. Through CDL, libraries maintain an “owned-to-loaned” ratio - the library circulates the exact number of copies of a title that it owns, regardless of format, and prevents users from redistributing or copying the digitized version.