Cropping the Ocean is a sustainable energy book. The book focuses on energy that is consumed at insignificant rates compared to its supply and with manageable collateral effects, especially environmental effects. A book is a set of sheets of paper, parchment, or similar materials that are fastened together to hinge at one side. A single sheet within a book is a leaf, and each side of a leaf is a page. Writing or images can be printed or drawn on a book's pages. An electronic image that is formatted to resemble a book on a computer screen, smartphone or e-reader device is known as an electronic book or e-book. The term "books" may also refer to a body of works of literature, or a main division of literature (e.g., children's literature) . In library and information science, a book is called a monograph, to distinguish it from serial periodicals such as magazines, journals, or newspapers. In novels and sometimes other types of books (for example, biographies), a book may be divided into several large sections, also called books (Book 1, Book 2, Book 3, and so on). An avid reader or collector of books or a book lover is a bibliophile or colloquially, "bookworm". A shop where books are bought and sold is a bookshop or bookstore. Books are also sold in some department stores, drugstores and newspaper vendors. Books can also be borrowed from libraries. Google has estimated that as of 2010, approximately 130,000,000 distinct titles had been published. In some wealthier nations, the sale of printed books has decreased because of the use of e-books, though sales of e-books declined in the first half of 2015. When writing systems were created in ancient civilizations, a variety of objects, such as stone, clay, tree bark, metal sheets, might be used for writing. The study of such inscriptions forms a major part of history. The study of inscriptions is known as epigraphy. Alphabetic writing emerged in Egypt. The Ancient Egyptians would often write on papyrus, a plant grown along the Nile River. At first the words were not separated from each other (scriptura continua) and there was no punctuation. Texts were written from right to left, left to right, or even so that alternate lines read in opposite directions. The technical term for that last type of writing is 'boustrophedon', which means literally 'ox-turning' for the way a farmer drives an ox to plough his fields. Steam-powered printing presses became popular in the early 19th century. These machines could print 1,100 sheets per hour, but workers could only set 2,000 letters per hour. Monotype and linotype typesetting machines were introduced in the late 19th century. They could set more than 6,000 letters per hour and an entire line of type at once. There have been numerous improvements in the printing press. As well, the conditions for freedom of the press have been improved through the gradual relaxation of restrictive censorship laws. See also intellectual property, public domain, copyright. In mid-20th century, European book production had risen to over 200,000 titles per year. 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.