Areopagitica is Milton's treatise, addressed to Parliament and printed in 1644, in favor of "unlicensed" printing, what we would today call freedom of the press. His case doesn't rest on a fundamental right to freedom of expression, however. Milton has a number of arguments, but his most powerful (and the most relevant to Paradise Lost) is that reading provides constant and necessary opportunities to test our virtue.
On the necessity of intellectual liberty: Many there be that complain of divine Providence for suffering Adam to transgress. Foolish tongues! When God gave him reason, he gave him freedom to choose, for reason is but choosing; he had been else a mere artificial Adam, such an Adam as he is in the motions. We ourselves esteem not of that obedience, or love, or gift, which is of force: God therefore left him free, set before him a provoking object ever almost in his eyes; herein consisted his merit, herein the right of his reward, the praise of his abstinence. Wherefore did he create passions within us, pleasures round about us, but that these rightly tempered are the very ingredients of virtue? They are not skilful considerers of human things, who imagine to remove sin by removing the matter of sin; for, besides that it is a huge heap increasing under the very act of diminishing, though some part of it may for a time be withdrawn from some persons, it cannot from all, in such a universal thing as books are; and when this is done, yet the sin remain entire. Though ye take from a covetous man all his treasure, he has yet one jewel left: ye cannot bereave him of his covetousness. Banish all objects of lust, shut up all youth into the severest discipline that can be exercised in any hermitage, ye cannot make them chaste that came not thither so: such great care and wisdom is required to the right managing of this point.
On the difficulty of contemporary ethical decisions: Good and evil we know in the field of this world grow up together almost inseparably; and the knowledge of good is so involved and interwoven with the knowledge of evil, and in so many cunning resemblances hardly to be discerned, that those confused seeds which were imposed upon Psyche as an incessant labour to cull out, and sort asunder, were not more intermixed. It was from out the rind of one apple tasted, that the knowledge of good and evil, as two twins cleaving together, leaped forth into the world. And perhaps this is that doom which Adam fell into of knowing good and evil, that is to say of knowing good by evil. As therefore the state of man now is; what wisdom can there be to choose, what continence to forbear without the knowledge of evil? He that can apprehend and consider vice with all her baits and seeming pleasures, and yet abstain, and yet distinguish, and yet prefer that which is truly better, he is the true warfaring Christian.
On the actively engaged ethical life: I cannot praise a fugitive and cloistered virtue, unexercised and unbreathed, that never sallies out and sees her adversary but slinks out of the race, where that immortal garland is to be run for, not without dust and heat. Assuredly we bring not innocence into the world, we bring impurity much rather; that which purifies us is trial, and trial is by what is contrary....
Reading as a form of ethical exercise: Since therefore the knowledge and survey of vice is in this world so necessary to the constituting of human virtue, and the scanning of error to the confirmation of truth, how can we more safely, and with less danger, scout into the regions of sin and falsity than by reading all manner of tractates and hearing all manner of reason? And this is the benefit which may be had of books promiscuously read...