Books are always points of departure to worlds unknown. Reading fiction opens up not only new understanding of people and new ways of thinking about life, but also a thirst for new facts.
Reading Charlotte's Web, for example, can lead kids to wanting to learn more about spiders, or pigs, or rats. (What fun to write a compare and contrast of Templeton in Charlotte's Web and Chiaroscuro in The Tale of Despereaux.)
On the other hand, reading a non-fiction book about spiders can inspire a kid to write a story about spiders. Or reading a book about mythology can inspire a kid to write her own personal mythology.
The important think is to catch whatever ignites a kid's interest and then pursue it down as many roads as possible. Go there and back again.