Milton: Books are not dead things, but contain a potency of life..

For Books are not absolutely dead things, but doe contain a potencie of life in them to be as active as that soule was whose progeny they are; nay they do preserve as in a violl the purest efficacie and extraction of that living intellect that bred them. I know they are as lively, and as vigorously productive, as those fabulous Dragons teeth; and being sown up and down, may chance to spring up armed men. And yet on the other hand, unlesse warinesse be us'd, as good almost kill a Man as kill a good Book; who kills a Man kills a reasonable creature, Gods Image; but heewho destroyes a good Booke, kills reason it selfe, kills the Image of God, as it were in the eye. Many a man lives a burden to the Earth; but a good Booke is the pretious life-blood of a master spirit, imbalm'd and treasur'd up on purpose to a life beyond life. 'Tis true, no age can restore a life, whereof perhaps there is no great losse; and revolutions of ages do not oft recover the losse of a rejected truth, for the want of which whole Nations fare the worse. We should be wary therefore what persecution we raise against the living labours of publick men, how we spill that season'd life of man preserv'd and stor'd up in Books; since we see a kinde of homicide may be thus committed, sometimes a martyrdome, and if it extend to the whole impression, a kinde of massacre, whereof the execution ends not in the slaying of an elementall life, but strikes at that ethereall and fift essence, the breath of reason it selfe, slaies an immortality rather then a life. 

John Milton, Areopagitica

Here is a useful summary of the argument that Milton makes in Areopagitica from the Encyclopedia Brittanica