Is the complexity of each book of the Bible from the mind of the direct author or authors, or is it from the mind of God?
You've asked two questions there, and the best answers I can provide you are...Yes and Yes.
2 Timothy 3:16 16 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof,
for correction, for instruction in righteousness:
As I read this verse I understand it to say that God told them what to write, through direct inspiration
to the writer. Still, we see the individuality of each author in their writing style, therefore God did not
always inspire/command them to write his words verbatim.
Paul knew what the issues were in each church, and he sought God's wisdom to guide him, and
God gave him that wisdom, telling him what to write of in his epistle. Everything that was written
was what God wanted written. Paul wrote what God told him to write, but in his own manner.
In 1 Corinthians 9, Paul, in his epistle to the Church in Corinth, says this:
24 Know ye not that they which run in a race run all, but one receiveth the prize?
So run, that ye may obtain.
Paul uses the term "run the race", as the people of Corinth can relate to it. The Isthmian Games,
which were sort of an early version of the Olympics, were held in Corinth and included many
foot races. That is Paul's personality reaching out to the people of Corinth.
In the Psalms we can see David's passion and love of the Lord, but also his deep pain written in a
manner that you can almost feel it in your chest as you read...
Psalms 6
2 Have mercy upon me, O LORD; for I am weak: O LORD, heal me; for my bones are vexed.
3 My soul is also sore vexed: but thou, O LORD, how long?
4 Return, O LORD, deliver my soul: oh save me for thy mercies' sake.
5 For in death there is no remembrance of thee: in the grave who shall give thee thanks?
6 I am weary with my groaning; all the night make I my bed to swim; I water my couch with my tears.
7 Mine eye is consumed because of grief; it waxeth old because of all mine enemies.
8 Depart from me, all ye workers of iniquity; for the LORD hath heard the voice of my weeping.
So, in asking
....Is the complexity of each book of the Bible from the mind of the direct author or authors,
or is it from the mind of God?
Yes, and Yes.