Eventually, a source of knowledge. For now, something to burn when you are freezing.
In-game description

A book is a material in The Long Dark.

A book is a fuel source for fire and increases the probability of lighting a fire by 35%. Books are generally found inside houses and are fairly common, especially in the larger, fancier homes like the Pleasant Valley Farmstead.

Books have an estimated fire duration of 18 minutes.

It is the worst time/weight efficient fire fuel in the game but works wonders when used to start a new fire.

Special research books exist in the game which can be studied to improve the player's skills.

Hides Black bearDeerMooseRabbitWolf
Other Crow featherGut
Medicinal Old man's beard lichenRose hipReishi mushroom
Sapling BirchMaple
Renewable Cedar firewoodCoalFir firewoodStick
Nonrenewable BookFirelogReclaimed wood
Renewable Birch barkCat tail headTinder plug
Nonrenewable CashNewsprintNewsprint rollStack of papers
Miscellaneous Arrow shaftArrowheadClothCured leatherScrap metal