The Bible includes books as well as letters. How important is that mail and who it was written to?

Bonus Questions
What letter, lost to us, does Paul tell the Colossians to also read? (Colossians 4:16)
Who wrote most of the letters that are in the Bible? (Books of the Bible)
What is the Greek word for letter? (Wikipedia)

Main Thought
The Bible isn’t just one book, it’s a collection of many books. It even includes letters written to specific people!