the aim of thucydides was to understand the spartan's political, social and military systems because they had beat the athenians. He IS biased twoards his native homeland, however he did greatly admire the spartans, his work was an attempt to learn from them so that the greeks could improve their own society.
classics_chic is right, plutarch IS out of his time period, however, it depends on who ur comparing him to: plutarch would still be considered a fairly contemporary, primary source even though he is writing after this time period - the distance between him and the times and modern writers and the times means that he is still considered a valuable primary source.
try lookin in the hist ext forum if u need more info - these historians r studied as part of the 'history and historians' course.