Book

Books are crafted from paper and currently serve no gameplay purpose except in crafting. Books and paper can naturally occur in chests, inside strongholds.

Books were added in Alpha 1.0.11. Their only use was to make Bookshelves, which was used as a purely decorative block. As of 1.9 pre3 (officially released in version 1.0.0), A book is also used to make an Enchantment Table.

Though it hasn't been confirmed yet, it has been mentioned that with the new code Notch made for the maps, he will be able to, in future updates, make books that players can type in and share with other players. The same thing can apply to custom paintings.

Crafting
Books are crafted from three paper sheets in a vertical column.