The prose style of this epistle is one that is based around the principles of rhetoric. Milton seeks to argue his point, which in this case concerns the need for freedom of speech and in particular the right to publish pamphlets without official sanction. He does this through the principles of rhetoric, using a number of different tactics and strategies in order to prove his point. Mythical allusions, rhetorical questions, sarcasm, anticipating opposing arguments: these are all essential ingredients of his prose style. Note, for example, the following quote, that comes after Milton has just spent two paragraphs describing how man is destined to live in a world where good and evil coexist:
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.
Note the way that Milton begins by referencing to what he has already shown in the prior paragraphs: man lives in a world where good and evil coexist, and therefore has to know what evil looks like in order to develop "human virtue." He then uses a rhetorical question, which assumes agreement with his point, to argue that the publishing of pamphlets forms an excellent role in the process of "scanning of error." Lastly, the use of the word "promiscuously" in the final sentence is of course sarcastic, as this is an adverb that is used in a derogatory fashion, and Milton uses it to attack his opponents who believe this, as he is arguing the opposite, saying that "books promiscuously read" can actually be very positive and helpful. Milton's prose style thus demonstrates excellent mastery of rhetorical strategies as he seeks to establish his argument and make his point.