Helen,
Moodle does not permit the addition of resources and activities to a Book resource or to a new Moodle page. I believe a good course design has all assignments and forums available on the front page so they are not hidden or embedded in other objects.
However, any resource or activity you create in a course has a web url. So you can do what you want to do - cross link to an assignment or forum.
1. Create an assignment or forum in the standard manner.
2. Right-click on the link to the assignment or forum and copy the link location. This usually works - if not, open the assignment or forum and copy the resulting link that appears in your browser address bar.
3. Go to the Book you are creating. Add text, such as "Upload your assignment here." Select the text and click on the editor link icon (chain link). Paste the copied url in the link box.
Floyd Saner