Is my ex wife entitled to proceeds from a book I publish after being divorced because 50% was written while I was married?

It is hard to say and you should consult with an attorney in person, but when you file a case information statement in preparation for a divorce one of the sections you fill out is to describe contingent liabililities and contingent assets. Contingent assets, for instance, can be law suits, patents, or other assets that are not in hand but may be expected in the future. Without getting into specifics, if you were just writing a book for your enjoyment while you were married with no intention or idea of publishing, you may be able to state that the proceeds arose after the date of the complaint. However, if you were under some contract or had some more definite hope of publishing your book during the time you were married, then she may have a claim to some of the proceeds. There is not really enough info to give a more definite answer.