Of course, some people try to equivocate and say, "Well duh, God didn't write it physically. He just inspired the prophets to write this or that." Well, sorry, but that is writing the Bible. You don't have to physically write a book with your hand in order to be the author of it. Stephen Hawking can't physically write with his hands, but he is the author of many books. Other authors have dictated their books and never wrote them by hand. If God gave the exact details that he wanted his prophets to write and they simply reported what God told them to say then we would attribute authorship to God, not the scribe who took down the words. Anyway, according to Moses God did write parts of the bible physically, as in Ex 31:18 which says, "When the LORD finished speaking to Moses on Mount Sinai, he gave him the two tablets of the Testimony, the tablets of stone inscribed by the finger of God." Throughout the books attributed to Moses it also contains statements that say, "I am the Lord" who did such-and-such, or "The Lord says..." do this or that. Later prophets like Isaiah use a formula where they say, "This is what the Lord says", and then they provide a verbatim quotation attributed to God. Sorry, but that sounds like more than just inspiring a vague idea.
Of course, if you want to reject this and admit that many prophets appear to heavily modify what God told them, injecting their own ideas in place of God's, and failing to fully and accurately report what God supposedly said to them then we could call it the work of man. Then God would not be the author of such works, though perhaps he could still be listed as a co-author. I think many religious people, particularly of the fundamentalist variety, would have a lot of problems with that position. However, since this Bible passage says that God couldn't have written the confusing parts, and most of the Bible is highly confusing to just about anybody who has ever read it, the only alternative is that human beings were the authors....unless you want to try the Muslim strategy, ala the Satanic Verses, and say that Satan wrote or inspired the confusing parts. After all Paul admits that he sometimes does bad things, even though he doesn't want to and claims it's the "sin living in me that does it". Remember that this is a guy wrote significant parts of the Christian New Testament, leading us to wonder if this might have happened when he was writing his various epistles. So, I'm not sure that this excuse really helps much. As with with much excuse-making, it just digs the hole deeper. Now we're not sure what we can believe from him and what we can't. If God is not the author of confusion then certainly Paul is.
BTW, this does raise an interesting legal issue. If God is the author of the Bible and God is immortal, then, under current copyright laws he should maintain exclusive ownership of this work and control over derivative works. However, again, perhaps no work of literature has been more infringed upon, in terms of authorship rights than the bible. Often, when a person blatantly refuses to assert his or her rights, despite overt, hostile, and notorious infringement upon those rights then this person can lose those rights by a process called "adverse possession". However, given that so many people have infringed upon these rights for so long, to whom should we award ownership? For the most part, we treat the bible as a work in the Public Domain, but it does seem funny that God would not even own his own work.