In the history of Homo sapiens, the book is an anthropological development, similar essentially to the invention of the wheel. While there are many reasons why written language might be considered more important generally, such as its permanence, level of formality, correctness, and highly organized style, linguists consider the spoken form of language to be of primary importance. Writing evolved from speech.
Some cultures have no written language.
Writing is a much later historical phenomenon. Writing centers in the brain have been found to overlay the centers concerned with speech production and comprehension. Finally, writing must be taught, whereas speech is acquired naturally without formal instruction. Consider arguments to support or refute why linguists consider writing to be of secondary importance.