Depending on the author 17th century English can also be very close to modern English. A couple phrases will be off and the spelling is different, but most of the difficulty is more the author using constructions that have fallen out of use or "showing off" with overly complicated sentences.

For example here's an excerpt from 1688's "Oroonoko"

  I have often seen and convers'd with this great Man, and been a Witness to many of his mighty Actions; and do assure my Reader, the most Illustrious Courts cou'd not have produc'd a braver Man, both for Greatness of Courage and Mind, a Judgment more solid, a Wit more quick, and a Conversation more sweet and diverting. He knew almost as much as if he had read much: He had heard of, and admir'd the Romans; he had heard of the late Civil Wars in England, and the deplorable Death of our great Monarch; and wou'd discourse of it with all the Sense, and Abhorrence of the Injustice imaginable. He had an extream good and graceful Mien, and all the Civility of a well-bred great Man.