A book was a written work. In some cultures and times, books took the form of an actual physical means of data collection and preservation composed of sheets that were bound together with a protective cover. The sheets of a book, the individual sides of which were referred to as pages, could contain printed, written or illustrated information. In some cases, all of the pages within a book were intentionally left blank so that its owner could record his or her own information within it for professional or private reasons.