Throughout the 20th century, libraries have faced an ever-increasing rate of publishing, sometimes called an information explosion . The advent of electronic publishing and the internet means that much new information is not printed in paper books, but is made available online through a digital library , on CD-ROM , in the form of e-books or other online media. An on-line book is an e-book that is available online through the internet. Though many books are produced digitally, most digital versions are not available to the public, and there is no decline in the rate of paper publishing.  There is an effort, however, to convert books that are in the public domain into a digital medium for unlimited redistribution and infinite availability. This effort is spearheaded by Project Gutenberg combined with Distributed Proofreaders . There have also been new developments in the process of publishing books. Technologies such as POD or " print on demand ", which make it possible to print as few as one book at a time, have made self-publishing (and vanity publishing ) much easier and more affordable. On-demand publishing has allowed publishers, by avoiding the high costs of warehousing, to keep low-selling books in print rather than declaring them out of print.