Book

Books may be shared either as a physical thing or (the contents) copied in digital form.

physical books

 * Bookcrossing (CDCrossing) - bookcrossers release books from their posession to freely share them with likeminded people

Books may also be shared via
 * give-away shops or via
 * The Freecycle Network

There are also projects for borrowing and lending books without giving up posession - to share only the contained knowledge, so to say. Besides specialized websites and networks they may also be shared via more general sharing networks. (e.g. Nutzigems)
 * distributed libraries (community of people borrowing books from each other) - see The Distributed Library Project
 * www.leihnetzwerk.de - an existing distributed library

electronic books
There are free content digital books that may legally be copied and shared freely. A big pool of public domain literature that is free content now because of expired copyright (normally 70 years after death of the author) is available. Apart from that some authors make their works available under free content licenses (e.g. Creative Commons licenses) rightaway. But also many copyrighted books get shared around as files, often illegally - partially after retrodigitization or even after breaking digital copy preventing mechanisms of books that are for example sold already as an electronic book.

There is a number of projects with collections of free content books:
 * Project Gutenberg - www.gutenberg.org
 * the Internet Archive
 * Zeno.org - www.zeno.org
 * Wikibooks - wikibooks.org
 * Wikisource - wikisource.org
 * The Latin Library - thelatinlibrary.com

There are also electronic books being shared over file sharing networks - also non-free literature. There is a scene of people who digitize (scan, OCR, maybe proofreading) existing (often also copyrighted) books and make them available for download. (So called "pirates"...)

Book reviews

 * GoodReads is a (social networking) website for people to share reviews on books and recommendations.