As a native English speaker, I would use a past tense to describe something that was once true but no longer is, and the present to describe something that is still true. Barack Obama was the president; Donald Trump is the president (as of the time of writing). Similarly, Julius Caesar was a conqueror, because he hasn't conquered anything for a few thousand years now.
I would do the same in Latin: Obama erat praesidens, but Trump est praesidens. The choice of perfect versus imperfect depends on whether I'm describing a state or an event; since being president lasts for a significant period of time, the imperfect feels more natural. On the other hand, Caesar perivit, not perebat, since I consider it a single event rather than a state of being.
The second example is a bit misleading: the full verb in English is "was written", since most passives in English are formed with a form of "be" with the passive participle. Latin sometimes does the same, but uses the tenses a bit differently: the direct equivalent would be scriptus est, with a present-tense form of esse. But the compound verb is still in the perfect tense, since the writing is over and done with now.