We’ll give you the answer for this question, but first let’s start with some history. Books appeared as method of knowledge transferring a long time ago. They allowed to take an accumulated experience of elder generations and give it to younger ones. Old books were written by specially educated people, the so called penmen. Information was stored on rolled sheets of papyrus. Later people have started to use parchment for such purposes. Sheets of parchment were collected into so-called bindings. Thus the first books appeared.
Parchment books were very expensive and their total number in the Middle Ages was sufficiently limited. Initially the main limiting factor was the speed of writing: all the text was handwritten in all copies. Even for specially trained person it took lots of time. Later, Johannes Gutenberg invented a movable type for bookprinting in Europe which noticeably accelerated books production. It had resulted in the new limiting factor: parchment was too expensive and also unsatisfactory for book printing. It was time when paper came in use. Paper was invented in China, approximately in the II century BC. It came to Europe in XII century AD. Active usage of it for books production has begun in old XVI century and holds till nowadays.
With time color of book paper becomes similar to color of a wood which was used for its production. This happens due to slow oxidation of the main wood component, lignine, and creation of colored structures by oxides.
These structures are removed during paper manufacturing using strong oxidizers like oxygen, chlorine dioxide or chloride of lime. Nevertheless the majority of lignine is saved. Thus, initially bleached, old paper anyway goes yellowish with time.
Nowadays total removal of lignin from paper is possible and can be performed using fungous microorganisms which process only lignin but not cellulose. Also some chemical methods are used. For example, acetic acid with small amount of sulfuric acid or zinc chloride removes lignin from paper. This technology allows to obtain lignin separately, for further usage in pharmacy, flawor manufacturing and some synthetical resins.