This exact argument can be made against laws banning murder. The murderer can argue that he never signed a contract not to kill anyone. Thus, this argument ALONE is not sufficient.
Murder is covered under the non-aggression principle. There are no contracts needed for that.
This exact argument can be made against third party resellers of items acquired by theft. He never signed a contract with the original party and should therefore be allowed to keep the stolen goods. Thus, this argument ALONE is not sufficient. Illegal organ sales for instance, should by this logic be completely legal.
There are only two legitimate ways to obtain property, by claiming unowned property or by buying it from the
current owner. A person that receives stolen property from a third party has done neither so they don't legitimately own it.
The same can be said about a slave trader. The third party who buys a slave didn't originally enslave the person. Now that slave is property and so third party slavery is ok. He never agreed not to enslave that party. Thus, this argument ALONE is not sufficient.
The argument against stolen property applies to slavery. I own myself and therefore only I can legitimately sell myself into slavery, which I see no problem with. If someone wants to sell theirself into slavery, have at it.
The only thing you prove here is that the labor argument ALONE is not sufficient. I need to specify that it only applies to peaceful work, i.e. work that does not violate the property rights of others. But whose property do I violate when I write a book? In what way is sitting in my own home writing a book on my own computer which I bought with my own money that I earned fair and square not peaceful?
Let's recap, shall we. You think that you own something because you mix your labor with it. I said that's false and I provided an example illustrating why. Then you said (I think) that you can mix your labor with a book and thereby own the book. No, either you already owned the book and writing in it is just doing whatever you want with your property or someone else owns the book and you just defaced their property. Writing in a book has nothing to do with owning the book. Of course, what exactly that has to do with owning what's written in the book is beyond me but it sounds like question-begging thus far.