The history of the "copyleft" movement is more or less coeval with the emergence of the Web as a means and mode of sourcing, creating, and publishing works, whether texts, images, music, or multi-media productions. Some trace the movement to the desire of some computer programmers in the 1970's to create versions of programming languages and software that could be developed independently of corporate-owned commercial products. Others see its roots in the xeroxed or mimeographed "zine" culture, or in pubk-rock designers' preferences for public-domain clipart. Whatever its exact origins, a signal moment arrived in 1988 when Richard Stallman created the first copyleft license, which he dubbed the EGPL (for Emacs General Public License), which evolved into the GNU General Public License in 1989. A version of this license was, until recently, used by the Wikipedia and many other wiki- and crowd-sourced sites; in essence, it declares that the material licensed may be shared by anyone as long as a) they indicate the source; and b) the same license to share is imposed upon all subsequent users. The GNU license originally didn't contemplate "remixing" in which the GNU-licensed material might be slight, nor did it account for problems that might occur when someone tried to copyright a longer text that quoted from GNU-licensed material. In 2009, in part because of such concerns, the Wikimedia Foundation dropped the GNU license in favor of a Creative Commons license known as CC-BY-SA, which has similar attribution and share/alike, but is friendlier to quotation/remix, and doesn't require reproducing the full text of the license in every work (Creative Commons maintains human-readable and legal versions of all its licenses).
Creative Commons licenses are now quite common, and have been used on blogs, text archives, wikis, streaming music and media, and downloadable media. I haven't yet seen one on a printed book, but it's certainly conceivable that it will be on certain e-books or other reference works. They offer the advantage of a variety of licenses, which allow for a) Copying; b) Modifying; c) doing either with or without attrubution; and d) allowing commercial or non-commercial use. Creators of content can thus exercise as much, or as little control over the re-use of their material as they like. Of course, no CC licensed material has yet been at issue in a courtroom, and as a result there is no case law to indicate how effective these licenses may be when it comes to the vital question of whether and to what extent they are enforceable. And perhaps that's best, for now.