If you've sold any of that material to someone else, you cannot republish it without their consent. You can likely cite portions of it as long as you attribute those portions to whatever medium the new owners used to publish it.
If you've already published the work, for example, a research paper, you need to cite the original work. Self-plagiarism actually does exist. This is especially important if you've modified the original work. This is also more of an ethical issue. Most people get away with this anyway.
If you haven't sold or published your work, you should be fine.