According to the answer key section the correct tense to use in the second sentence is Past Simple. I wonder whether it is possible to use the Past Perfect tense form "had written" here. Please, give reasons if possible or not.
On January 10, 1845, Robert Browning, a little-known poet and playwright, sent a letter to Elizabeth Barrett, an internationally renowned poet, an invalid, after reading her volume of poetry. Over the course of the next 20 months, they wrote/had written each other close to 600 letters.
What if one views this situation outside the time-reference context provided? Meaning if one emphasizes the fact that 600 letters within that period of time is quite a big number. So, the focus in this case is on the result (the number of the letters). By now they are clearly dead. Their correspondence took place while they were alive long time ago. Their lives are part of the past already. Their '600 letters' within 20 months can be viewed as some result.