I am negotiating an agreement with a company to produce some online training videos for them. The agreement states that the training videos will be considered a "work for hire" and the company will own all rights to the materials in the videos. I have published five books and it is very likely that I will use material from some of my separately copyrighted works in producing these training videos. While I am in agreement with giving the company ownership rights to the videos I produce I don't want to give up ownership rights of some of the separately-copyrighted material that I might use in the videos. How can I best accomplish this?